Given the pace of technology changes, there are lots of examples with computing.
Computer Programs and Hardware
Internet portals like Prodigy, CompuServe, iMagination, etc. They were called portals because that's how you usually entered the Internet — they had a lot of links to useful sites, news and a content listing. When the Internet was fledgling during the '90s, they were extremely popular. However, the more efficient, less resource-intensive, and free World Wide Web put them on a steady decline. Today, America Online is the only one of these services that still remains, and even that's pretty much on its last legs, remembered as a symbol of all that was wrong with the mainstream internet in the '90s.
Floppy drives and floppy disks (commonly known as the A:\ drive to Windows usersnote DOS and Windows are the only operating systems to label drives this way) are mostly dead thanks to the fast advancements in computer and storage technology. The most basic CD or DVD simply has capacity orders of magnitude larger than the floppy disk and DVD and USB drives perform much faster than a floppy drive.
Funnily, the aforementioned rise of the CD-ROM format started a five or six year trend that would itself become this: the so-called "Multimedia Revolution." During the early-1990's, as the CD-ROM format was becoming mainstream, "multimedia" had become a huge buzzword in the computing industry. The idea was that basic domestic activities, from mothers looking up dinner recipes to teenagers researching the first World War, could now be done on home computers thanks to the large amounts of information CD's could store. Between 1991 and 1996, numerous "multimedia applications" were developed, from Microsoft Encarta (a full-fledged CD-ROM based encyclopedia) to so-called "edutainment" games for children such as Putt-Putt Joins The Parade.
Two things effectively killed the "multimedia" movement. The first and most obvious was the rise of the World Wide Web, quickly rendering CD-ROM based reference software obsolete. The other was the rise in Microsoft Windows 95 as a gaming platform. Originally, Microsoft was pushing Windows 95 as the "ultimate multimedia powerhouse", with Microsoft Encarta being its big Killer App. Around that same time, a little game called Doom was released. After audiences turned out to be far more intrigued by Doom than Encarta, Bill Gates gradually phased the "multimedia" focus out of his promotions of Windows 95, instead emphasizing its potential as a gaming platform, which was heavily fueled by the creation of DirectX, a game development library that made computer game development and installation easier than it ever was on DOS (no longer did gamers need to mess with confusing soundcard configurations or expensive memory managers to get a single game running - DirectX took care of all those issues automatically). Today, the "multimedia movement" is seen as a joke and little more than a footnote when discussing the rise of the personal computer.
Many Windows operating systems are obsolete due to newer, faster, and more secure versions being released every few years. Windows XP is still widely used by schools and many workplaces—including many governmental units (many a new hire at a US federal courthouse is surprised to find the computer runs XP)—and it is also still used by consumers who do not wish to upgrade their OS, even though Windows XP is no longer supported by Microsoft. Some places may still use Windows 95/98/2000 as well just for their simplicity (and thus making certain work tasks quicker to do without having to go through more complicated software), but those are incredibly rare to see.
Having certain e-mail providers is seen as sure signs that you are an old person who probably barely uses the internet. These include Yahoo, Hotmail, AOL, and various discontinued internet providers like Netzero. In fact, pretty much any e-mail provider other than Gmail seems to be slipping into this trope.
However, Hotmail may end up seeing a second life as Outlook (since the Hotmail accounts were grandfathered when the change occurred).
WebRings, Topsites and internet directories have been affected by this trope, with the prominence of Google (which most discounts their effects on a site's SEO) and the rise in social networks as a means of promotion having made things like giant lists of links and the like kind of useless. Indeed, some might say the only people who even visit such sites nowadays are webmasters trying to promote their work (instead of end users trying to find new content). This can be seen by how the Open Directory Project/DMOZ has slowly fallen in relevance, and even completely been dropped as a feature by Google in recent years.
Dedicated instant messenger programs (such as ICQ, AOL Instant Messenger, and MSN Messenger) for computers were very popular among Internet users during the days when a significant percentage of them had dial-up connections, as they would allow you to see when a friend logged on and instantly open a chat with them. But the rise in popularity of text messaging, cell phone plans with free long distance calling, social media sites like Facebook adding built-in chat features, gaming clients like Steam and Battle.Net having built-in instant messaging, and free video chat programs like Skype and Tango have caused dedicated instant messengers to become somewhat obsolete. These days, IM programs are mainly used by businesses as a way of allowing quick, easy communications between employees who may not be sitting right next to each other.
For that matter, the concept of meeting and chatting with total strangers on the Internet seems to have become this, now that issues like cyber-pedophilia and phishing have soured the once novel concept of meeting different people from around the world with just your computer.
GeoCities, which allowed many early denizens of the 'Net to make their own Web pages without needing to know how to use HTML. However, Sturgeon's Law was in full force, as seen in this article: "It didn't take long before this simple change altered the face of the internet. GeoCities gave everyone a place to call home, and then proved that most of us don't really have a lot to say. It didn't take long before GeoCities became home to the bottom of the Internet. Crackpot theories. Inane ramblings. Worm distribution." and "I think that most people set up a GC page as a novelty and then abandoned it leaving a whole lot of cyber-trash behind. That kind of ruined the overall GeoCities vibe; it wasn't long before you had to muck through a few dozen one-offs to find a page that was regularly maintained and had good, interesting content." GeoCities was often seen as a haven of garishly colored pages full of blink tags and animated GIFs. Also, the rise of blogging, as well as social media like Facebook, rendered the concept of a free personal homepage obsolete, while those who still wanted to build their own Websites moved on to more advanced tools.
By proxy, comment sections in major news articles and editorials significantly dwindled during the last three or four years. The initial idea of comment sections was that anybody could give his/her own commentary on whatever major social issue or event was being covered in the article. Unfortunately, as with GeoCities, Sturgeon's Law kicked in — for every one comment that was reasonably thoughtful and well written, you generally had to wade through at least nine or ten utterly worthless comments written by trolls, spammers, religious fanatics, political extremists, raging atheists and all-around raving lunatics. Needless to say, this was especially true when it came to articles related to volatile subjects such as politics or religion; a common joke was that the comments sections on such articles could resemble the ramblings on Stormfront, a prominent white supremacist message board. Thus, many websites have recently dispensed with comment sections altogether, while others have enforced stricter policies on what can and cannot be posted in them.
Usenet newsgroups have been a cornerstone of the Web since the 80s, and many are still active to this day. However, their relevance began sliding as early as the Eternal September of 1993, when America Online opened up Usenet, thus flooding many a group with college freshmen who had no netiquette to speak of. Over time, many forums withered and died as they became overrun with trolls, spammers, and the like. In addition, many of the "alt" groups had begun posting porn or pirated software, thus causing some ISPs to block off Usenet entirely. Also helping Usenet's demise was the rise of the Website and especially the Web forum, which allowed the same open discussion as a Usenet group, albeit with moderation to keep the Garbage Post Kids out.
vBulletin. Back in the early 00s, this was THE forum script of choice in the internet world. Basically, once you bought a vBulletin license, you knew you had the best possible software and were respected pretty much internet wide for it. And the creators had even better reputation. Then Internet Brands bought it. And released vBulletin 4. And the incredibly poor quality vBulletin 5. The software's popularity dropped sharply off a cliff and never recovered.
A lot of CMS systems suffered this too. PHP Nuke has basically fallen off the map, Mambo went from a big name to abandoned in a few years and even some of the more 'popular' scripts like Joomla and Drupal have become a lot less popular in recent years, having basically become unknown in large parts of the internet. This was mostly because Word Press went from 'blogging software' to 'quasi CMS with a near monopoly over the market', causing its rivals to pretty much just vanish or die out.
The MIDI format, even outside of video games, has fallen under this trope (see the video game section for more information). Back in the mid-late 1990s, MIDIs were often used as a cheaper and less memory intensive alternative to music. However, by the 2000s to the early tens, with the rise of MP3s and other sound formats becoming more popular (not to mention more economical, with hard drive sizes increasing and DSL/Cable modems speeding up file transfers), they are now usually subject to mockery.
The dot-com boom in the late '90s produced a great many Websites and companies that were built seemingly entirely on hype in business journals and kitschy adverts, among them Pets.com, Lycos, and Freeinternet.com. After the bubble burst in 2000/01 and dragged down many e-businesses with it, they went from being the subject of public admiration for their founders' ingenuity and entrepreneurial spirit to mockery for their folly virtually overnight. This E-Trade Super Bowl ad from 2001 (a parody of the Crying Indian ad) demonstrates the fall from grace that many such sites underwent.
"Mature humor" Flash sites - like Newgrounds, Camp Chaos and Icebox - were seen as the next big thing in the late '90s. At the time, seeing something so independent and uncensored was absolutely phenomenal, especially to younger teenagers. Once the millennium hit, though, the popularity of these sites took a downhill slope. Some of them got swept up in the economic dot-com bubble burst and shut down, others tried avenues into other forms of media and didn't have much success. As the scope of the Internet grew as well, some people just found the humor too juvenile to enjoy anymore. Newgrounds is still one of the few sites from then still doing well, and even they have tried to shy away from the tasteless humor aspect, instead mainly showcasing more art-oriented submissions.
As late as 2007, YTMND was one of the most powerful forces on the Internet - the day it managed to coordinate a raid among Something Awful, 4Chan, and Newgrounds remains one of the most astonishing achievements in trolling history. These days, it has an Alexa rank of about 35,878, and seems to risk a shutdown every other month. This is largely due to the novelty of a bunch of GIFs filling up the screen with looping audio having worn off in the wake of sites like YouTube, which offers far more substantial content.
Related to GeoCities above, dedicated fan pages seem to have gone the way of the dodo. This could probably be attributed to a saturation of such pages during the late 1990s, the fact that most of them were pretty much indistinguishable from one another (piles of mundane information about the interest, combined with some poorly-sequenced GIF images and cheesy MIDI music in the background), and the rise in professional media-centered sites like IGN and The AV Club giving people little reason to still make dedicated fan pages.
Digibutter.nerr. Launched as a forum about Super Paper Mario when the game was new, it gradually faded into irrelevance as the game lost steam, before eventually dying off for good when the owner converted it to a different forum script. Now all that remains are RSS feeds from popular sites and the odd comment from someone who'll never return again.
A couple of admin sites have come under this as well. Admin Fusion for instance was a rather successful site a couple of years ago, but multiple changes in ownership and a complete failure to manage the site have left it a spam ridden ghost town. Similar situations have been noted of Admin Addict and Top Admin, both of which were fairly popular or respected once and are now either shut down or completely dead.
Digg was a popular social news site, but a poorly-executed redesign caused most of its userbase to flee to Reddit. It was purchased and relaunched by Betaworks, but is a shell of itself.
The Game Overthinker episode "Who Will Be Remembered?" is basically a discussion of this trope, asking which iconic video games and characters will stand the test of time. Past examples from film, animation, comic books and stand-up comedy are employed to demonstrate how the trope works.
The episode "Thing We Lost in the Fire" also covers this trope, talking about how arcades have experienced this in the United States, and how they could possibly make a comeback (using the Golden Tee series of golf games as Exhibit A).
Discussed again in the episode "Setting Sun", where he talks about the decline of the Japanese game industry.
"Tribute" videos on YouTube. Back in the heyday of 2006-2009, it was common among the younger users to make slideshows/video collages of something, usually a sports team or fictional character, set to a song (usually Linkin Park, Green Day, Simple Plan, Three Days Grace or similar). Nowadays, with platforms such as Tumblr where people can blog and discuss such things more extensively, such videos are now considered obsolete or redundant. It didn't help that the videos always had a Hatedom among the more "mature" users, who found them narmy and clichéd. An easy way of telling if a YouTube user was under the age of 17 was by whether or not they had a Sasuke tribute set to Animal I Have Become in their uploads.
Same goes for AMVs, especially anime ones. Furthermore, the heyday of AM Vs was in the mid-to-late aughts, when YouTube was still fairly permissive of copyright. With YouTube becoming increasingly litigious in its pursuit of infringers, AMV makers are doubly at-risk - will they get flagged by a record company, or by the animation company? Consequently, thousands upon thousands of AM Vs have been Lost Forever and their makers banned since the crackdowns, leaving the practice unappealing.
Text-based video game countdowns (such as, say, Top 10 Easiest Bosses) were huge back in 2008 and people with those videos had views past the 10k mark. From 2010 onwards however those type of countdowns have faded out in favor of vocal countdowns and the people that made text based countdowns in the past are nowadays for the most part left in the dust (music based video game countdowns are still generally text based though).
Another thing to mention is the fact that many video game countdown artists only tend to play the popular games, which makes sure that one countdown of the same topic always overlaps with another. It can easily happen that a topic becomes this mainly due to the fact that you have seen all the stuff on there on other lists and thus countdowns with the same topics risk to get rendered obsolete by this phenomenon. Perhaps that is one of the reasons why many of the popular countdown artists (such as The Autarch) are always trying to search for new and original topics to cover.
The Star Wars vs. Star Trekdebates became this. One of the oldest discussion topics on the internet, it once had its own forum on Usenet, and countless Cross Over stories were written by the topic. The debate began to falter after various scientifically-oriented fans started crunching numbers, and came to a conclusion suggesting that a lone Star Destroyer could probably solo the entire Federation fleet - to say nothing of the massive disparity in resources between the two traditional sides (the 150-planet Federation is a whole two tiers below the million-planet Empire on the Kardashev Scale). Debates over the veracity of these calculations were then squashed by official material that provided statistics that were, if anything, higher than the usual claims. Since then, any given debate between the two sides tends to deliberately ignore the difference, analyze unusual situations (Voyager is replaced by a Star Destroyer, what happens?), or go for humor, with many of the traditional forms having been salted. Most importantly, George Lucas has gone on record by saying that Star Wars owes part of its success to the path that Star Trek help pave. Thus, he considers the rivalry absurd.
Because the lifecycle of a meme is rarely cyclical and often quite rapid, just about any Discredited Meme qualifies. Examples include: "I used to be an adventurer like you, then I took an arrow in the knee.", "The Harlem Shake", and many others. As the internet becomes more mainstream and interconnected, the shelf life of memes becomes shorter and shorter. People declared the Harlem Shake a dead meme after less than a month of it gaining prominence.
TFWiki.net is well-known for it's many running gags. It has a section in its policy page titled "Captions and jokes we're sick of", which lists all of the sites running gags and memes that have died out. Some of said jokes have become so despised by the userbase that mentioning them can get you a revert or even an editing ban if the mods aren't in a good mood.