A trope which has gone way beyond being a Discredited Trope to where the very act of parodying and/or subverting that trope has itself become a trope.
The progression is generally:
Clever idea → Trope → Discredited Trope → Dead Horse Trope.
→ Then, if the downward slide continues, it may end up as a Forgotten Trope.
Named for "Beatingnote or 'Flogging' a Dead Horse" - an old idiom that describes continuing in a course of action which is clearly pointless. For instance, there's no use in whipping your dead horse to make it move faster — that horse isdead; it's not going anywhere.
Naturally, the Dead Horse Tropes tend to be The Oldest Ones in the Book, too.
If a Dead Horse Trope is still used straight in recent works despite seemingly being subverted or parodied to death, it's an Undead Horse Trope. If it was never really played straight in the first place (but everyone assumes it was), it's a Dead Unicorn Trope. If it's so natural to the medium of storytelling that it can still be played straight no matter how often it's used and abused, it's an Omnipresent Trope. If the trope not only makes viewers/readers groan but also makes them angry, you've probably got a Pet Peeve Trope.
A common cause of Seinfeld Is Unfunny, because it's hard to imagine yourself back into the innocent frame of mind when this was new. A related trope is Deader than Disco, where something that was very popular in its day later becomes better known for the backlash against it than for its own merits.
Please put any examples on the trope pages, as it gets in the way of indexing.
America Saves the Day has been one since the seventies at the latest; the Vietnam War and an overdose of this trope in the Hollywood of the forties, fifties, and sixties conspired to kill it. This trope is at the point now that when it's not subverted it still feels like a subversion.
Bank Toaster: Banks do not give new accounts items directly, instead using a points reward system or "cash back" to let customers obtain their own custom prizes.
Black Dude Dies First: After comedians started mocking its use, and general racism decreased. It still occasionally happens but it's not expected to anymore, to the point that most examples are coincidences in modern works (or intentional parodies)...most.
The Dark Age of Comic Books: While many series have continued to get Darker and Edgier, the specific stylings of 90's comics, such as pouches, random metal ornamentation, and bizarre headpieces have all been parodied well after they went out of style.
The excuse "A dog ate my homework!" is so prevalent in media that even students know better than to pull this when they don't finish their homework. Nowadays, this is replaced by the more plausible "My printer stopped working!" The logical extension of this trope, "The dog ate my printer!", has yet to catch on.
Dunce Cap: Actual schools don't do this much anymore.
Face on a Milk Carton: Thanks to, in the U.S. at least, the Amber Alert system which allows missing children's names to be broadcast on television or on expressway signs within minutes. You might still see it on Iceland-brand milk bottles, but otherwise this is almost never seen anymore in real life, and hardly ever done seriously in fiction these days.
Flashback Stares: Characters who stop to stare wistfully into the mid-distance are more likely to be interrupted by another character asking what they're looking at.
Good Angel, Bad Angel: Straight examples are rare in these days. Parodies and subversions are more common: two devils, two angels, convincing devil and dimwit angel, the devil and the angel actually agree, etc.
Great White Hunter: A combination of accusations of racism and of an increase in animal rights. If one such hunter does show up, he'll either be laughably behind-the-times or genuinely evil.
The Greedy Jew stereotype in the western world, after it was used as a justification for the Holocaust.
Look Behind You: including the subversion of the villain refusing to look behind, lest the hero gets away, only to find that the hero was actually right. This is mainly for the fact that a lot of problems could be avoided by simply stepping aside, so that the villain could eye both the hero and whatever was behind him simultaneously.
Minstrel Shows: While it would be a stretch to say that racism is completely extinct, the more overt displays of it are certainly no longer generally acceptable.
90% of Your Brain : It turns out that most people do indeed use all of their brain, just not all at once the same way you won't have every light in your house on if you're not in a particular room. In fact, if you ever find yourself with all your "lights" on at once, congratulations: you're having a seizure.
Prince Charming: Time was, every fairytale had a character whose main function was to be a) physically attractive and b) a socially advantageous marriage prospect for the hero/heroine, by virtue of being wealthy and/or a member of the aristocracy. Information about this character's actual personality tended to be sketchy at best, except period dramas. Remained popular through the early Disney Animated Canon. Nowadays often subverted as being clueless and vain, if not downright evil. See Prince Charmless, which is the current form nowadays. See what we meant by the spoof becoming a trope?
Red Wire Blue Wire: 9/11, The Troubles and 30 years of terrorists hiding grenades inside dead cows and cars instead of metal boxes marked "BOMB" have stomped this one flat.
Standard '50s Father: The '50s ended, and took the more overt forms of patriarchy along with it. The Bumbling Dad, originally a pretty clear way of rebelling against this trope, eventually become so prevalent in modern media that showing a sensible, competent father (in a sitcom, anyway) is practically a subversion in itself.
Stern Nuns beating students at Catholic school. At least in the United States, nuns stopped doing this a long time ago (and also a good many Catholic schoolteachers are lay people), and yet Hollywood and TV shows constantly act like it's still standard procedure, as sitcom parents often threaten their rebellious teen with a transfer to a school run by nuns, implying that they will be beaten.
Take Me to Your Leader: Despite the fact that there is some Truth in Television in the sense that an explorer in a distant land, upon meeting some of the locals, might wish to speak with whoever's in charge around here, the form of this where visiting extraterrestrials request this is almost never played straight these days. This is usually replaced in most fiction (mainly movies) with We Come in Peace — Shoot to Kill- the alien captain will simply vaporize everyone in the room the moment someone says "hello," or he and his unit are mindless Starfish Aliens with no concept of diplomacy, or the local General Ripper will ignore protocol and blast a hole in the alien's chest long before any leader is summoned. When this line is used, it's generally combined with an "Always Wanted To Say That"-type line.
Vampire Vords: Strongly associated with "old", Bela Lugosi-style vampires. Neither the modern "sexy" vampires or the few genuinely threatening ones in modern fiction talk this way, because it's silly.
Working on the Chain Gang: The punishment was once very commonplace in Southern US states up until the mid-1950s, with Georgia being the last to abolish it in '55. Today only a single county in Arizona remains as the one place that still makes use of chain gangs, although inmates serving on these ones aren't shackled together anymore. Nowadays, chain gangs mostly just exist in period pieces in media that involve prisoners in the early half of the 20th Century. Replaced with "community service" nowadays; usually a crew of guys filling potholes on the highway or picking up litter in the park, but these activities aren't gritty or sexy enough for Hollywood so they rarely show up in media.