Magic: The Gathering has this in spades. The game's original setting was much closer to a Standard Fantasy Setting. (In fact, the first set, Alpha was a deliberate attempt to cram as many familiar fantasy elements as possible in one set.) The colors were much less defined mechanically than today - many cards did things that would be unacceptable in their colors today. The rules were messy. There were bizarre mechanics like flipping cards over in the air, dividing creatures into two different groups that can't ever meet, camouflaging creatures, subgames and playing for ante. Rules text was written in a much less formal style, the ultimate example of this probably being Rock Hydra. Also, some early cards also referred to abilities as "special powers." And finally, the "Block system" of one large set followed by two related sets, as we know it today, didn't actually begin until Mirage. Homelands was originally shoehorned into an Ice Age "block", but then later made Coldsnap to properly complete the Ice Age "block".
This can look especially weird with elements that have always been a part of the game, but didn't always have their modern rules. The most egregious examples are cards that represent weapons and the like before Equipment was made into a rule. With modern Equipment, a sword being used by a creature that gets killed can be equipped to a different creature. But old Equipment-like cards usually followed the user to the grave. This can also be seen in Arabian Nights, which has a lot of cards for unique characters (Aladdin, Ali Baba, etc.), but there was no "legend rule," so you could have a game with a half-dozen Aladdins running around at once!
This is particularly awkward when a long-standing ability finally gets keyworded, but not exactly how the previous version(s) worked. One example is "Deathtouch," which causes any amount of damage deal to a creature to be lethal—there were at least three previous versions, none of which worked that way (the other versions were all triggered abilities, either keying immediately off damage, immediately off blocking/being block regardless of damage, or at the end of combat after blocking/being blocked). The other primary example is "Lifelink," which causes the source's controller to gain life whenever it deals damage (like deathtouch, this is a property of the damage, not a triggered ability). The old version, usually referred as "Spirit Link," after the card that granted it, is a triggered ability. More importantly, "Spirit Link" causes whoever controls the effect granting "Spirit Link" to gain life, while Lifelink causes the controller of the damage source to gain life. This is relevant because a player can put "Spirit Link" on an opponent's creature to effectively nullify damage to themselves (as long as they don't die between the damage being dealt and the lifegain trigger resolving).
On the topic of "equipment" artifacts predating the actual Equipment mechanic, the one that worked most similarly to the modern concept was Zelyon Sword. It used tapping and a continuous targeting effect to stay "attached" to a creature. Also of note is that it was a rare that was effectively a 3-mana artifact that granted +2/+0 with equip 3. By contrast, one of the first "proper" equipment in the game, Bonesplitter, granted the same stat bonus for only 1 mana each to cast and equip, at common.
In fairness to the designers of Zelyon Sword's era, Bonesplittler is acknowledged to be a bit overpowered. However, it's less overpowered than Zelyon Sword is underpowered.
Also, many of the earliest cards had to basically explain the mechanics of the game in the card itself, with examples and all, while nowadays these are nearly always left out because the templating is much more streamlined and the rules are much more codified, with whole sections on things that used to be (and in a few cases still are) specific to one or two cards. A good example of this is Keldon Warlord, whose original card text was:
The Xs below are the number of non-wall creatures on your side, including Warlord. Thus if you have 2 other non-wall creatures, Warlord is 3/3. If one of those creatures is killed during the turn, Warlord immediately becomes 2/2.
The modern text is simply: "Keldon Warlord's power and toughness are each equal to the number of non-Wall creatures you control." Another good example is Control Magic, whose card text states:
You control target creature until enchantment is discarded or game ends. You can't tap target creature this turn, but if it was already tapped it stays tapped until you can untap it. If destroyed, target creature is put in its owner's graveyard.
The modern text just says: "You control enchanted creature."
This wording contains several other examples of Early Installment Weirdness. First and second, modern cards don't use the words "target" and "discard" the way this wording does. Third, the "can't tap" bit - an attempt to describe summoning sickness - was never entirely correct, so besides being redundant by modern templating standards, it's also just plain sloppy by those same standards. Fourth, the "until game ends" bit, while it meets with a resounding "well duh" from the modern-day reader, is there because at the time, Wizards of the Coast thought of ante as one of the game's most important rules. Therefore, they didn't think returning the card to its owner at the end of the game was nearly as obvious as it seems now. In reality, of course, ante was the first rule most people ignored.
While reminder text has always been a part of the game (look at early versions of Fog, for example), it was originally indistinguishable from rules text. Only in Mirage was it first put into parentheses and italics or used to define new or seldom-used keywords.
For a single-card example, here's a 2013 printing of Sol Ring with a silver card frame, rarity-coded expansion symbol, modern card frame and wording, and flavor text filling in the empty space in the text box. Here's the original with its old-school fantasy look, brown artifact frame, "mono artifact" on the type line, no expansion symbol, and giant rules text with no tap symbol that references "interrupts". The artist interprets Sol Ring as some kind of cosmic/stellar event instead of a wearable artifact, hence the modern flavor text referencing the older depiction as a lost artificer's technique from another time.
Warhammer 40,000 was initially just Warhammer FantasyRecycled In Space with things like the Eldar being explicitly called "Space Elves". It also wasn't actually called "Warhammer 40 000" untill the 2nd edition (the first edition is called Rogue Trader)
Of particular note is the fact that in its earliest incarnation 40k was more than a little campy and silly at times, whereas now the setting is famous for being a Darker and EdgierCrapsack Universe. Among the fandom, First Edition Warhammer 40k is usually only considered canon from a Broad Strokes point of view since large portions of its background information is no longer compatible with the current 40k universe.
The earliest edition of Warhammer 40000 was also missing some concepts that are now critical to the setting, the most obvious of them being daemons. There were still monsters in the Warp, but they weren't actually the now-iconic daemons.
The 1st edition of 40k had a lot more RPG-ish elements than the later ones, including needing a Game Master. The game was designed more for a small-scale squad-based combat rather than large armies. The game didn't become a full-fledged wargame untill 2nd edtion (although even then the rules were designed more for smaller-scale battles, making larger battles very time-consuming and rather awkward to play).
And then there's the art style. Behold the might of the Tyranid menace◊ and the terrifying◊ Carnifex! And stop snickering!
Space Marines started out being depicted with slogans printed on their armour that, as White Dwarf put it once, "would embarrass an American GI". if contemporary Space Marines have slogans on their armour, it's usually Latin, and one with the older ones would be given a stern talking-to by the Chaplain.
Don't forget the fact that in early Warhammer 40K THIS was considered canon. Any recent Space Marine or Inquisition would immediately brand Illiyan Nastase as an abomination and kill him.
Warhammer Fantasy Role Play, the role-playing game spinoff of the wargame, wasn't renewed for some fifteen or twenty years, thereby preserving a lot of early canon (like several never-seen-again races such as the Fimir, or the Slann being the Precursors themselves) in places where the WFRP was popular.
Dungeons & Dragons in its earliest incarnations included things that would confound players of later editions, such as a Level Cap and limited class selection based on race, the Exceptional Strength rule where Warrior classes (e.g. Fighter) could have a strength "between" 18 and 19 (18/59, for example), and Armor Class getting better as it got lower. If the ancient "Blue Book" were shown to a player who picked up the game after the mid-nineties, most of it wouldn't make sense. Generally, First and Second Editions are fairly similar, while 3.0 and 4th Edition were both radical departures from what came before. Some people like the old weirdness; each change has had caused the Unpleasable Fanbase to adopt every attitude conceivable from quitting the game to playing with 30 year old books to eagerly embracing every new rule as soon as it's published.
Third and Fourth Edition Dungeons and Dragons actually take their numbering from Advanced Dungeons and Dragons, as the two games were folded together after AD&D Second Edition was discontinued around 1999, and the Basic Dungeons and Dragons game was discontinued around that same time.
The very first edition of Dungeons and Dragons was actually an outgrowth of a tabletop wargame called Chainmail, created in the late '60s by Gary Gygax and Steve Perrin. The very first books required possession of the Chainmail game and the Avalon Hill game Outdoor Survival which was used for overland adventuring. They also had a number of elements taken from Lord of the Rings, including Hobbits, Ents and Balrogs, at least until The Saul Zaentz Co., J. R. R. Tolkien's literary agents, got wind of it and demanded the references be taken out (and by "taken out," we mean that there are now Halflings, Treants, and Balors - the more things change the more they stay the same).
The Original Edition rules actually recommend "At least one referee and from four to fifty players can be handled in any single campaign, but the referee to player ratio should be about 1:20 or thereabouts." This of course is because players were assumed to drop in and out of play of a given campaign, with the party composition differing from session to session, not because fifty players were expected to play at the same table.
Electrum currency in D&D, AD&D and AD&D 2nd edition was ignored in third and fourth edition, but make a return in fifth edition.
THAC0 in AD&D 1st and 2nd edition, when lower armor class represented more powerful armor. Done away with by third edition.
The first edition had the concept of potion miscibility, where combining two different potions (whether by mixing them or simply by drinking two potions in close proximity) could produce a number of effects, ranging from one of the potions' effects becoming permanent to the mixture blowing up — which was pretty much instant death if it happened in a poor player's stomach.
Vampire: The Masquerade was a vastly different beast in First Edition (1991-1992) and the earliest parts of Second Edition (around 1992-1993, though 2e as a whole continued until 1999). The biggest example of this is the total lack of a Metaplot. There were other key differences between First Edition and the later editions of the game.
The Sabbat were less of a more evil counterpart to the Camarilla and more of a mysterious boogeyman that you did NOT want to meet (The few Sabbat characters mentioned before 1993's Guide to the Sabbat had Humanity 0, high discipline levels, and had Sabbat listed as their Clan instead of Sect). In fact, the main conflict in First Edition and early Second Edition was between the Camarilla and Anarchs.
The player characters were assumed to be either Anarchs or have Anarch sympathies
There were two heroic vampire sects: The Children of Osiris and the Inconnu. Neither were mentioned ever again in Second Edition, and Revised Edition outright killed off the Children of Osiris in their mission to be Darker and Edgier than previous editions.
The Independent Clans were sidelined and marginal, and assumed to be extremely rare.
The other denizens of the Old World of Darkness were less defined. Werewolves and Mages were insanely powerful boss monsters whose motives and backgrounds were completely unknown. Ghosts and Faeries were also alluded to, and Faeries were even more dangerous than either Werewolves or Mages to the point that they did not have concrete stats, only suggestions for the GM. Mages were less about reshaping reality and more like traditional wizards with insanely high levels of Thaumaturgy and other spells.
Related to the previous note, the Antagonists section of the 1e Corebook emphasized that humans were the biggest threat. The majority of the antagonists in the section were human (mostly law enforcement and military personnel, although you still had traditional Vampire Hunter characters as well). In Revised Edition, the only human antagonists outlined were Vampire Hunter characters who already knew about the supernatural, specifically the Inquisitor, the Government Agent, and the Arcanum Scholar.
GURPS Fantasy wasn't originally a generic sourcebook. It instead presented a specific fantasy setting. By the fourth edition, GURPS Fantasy was revamped to work in any fantasy campaign, and the older material was continued with GURPS Banestorm.
The game was originally called Battledroids, but someone else owns "droids"). It started out with a much darker background than its later material - BattleMechs were literally irreplaceable, along with all the interstellar technology being irreplaceable. ComStar, the cult that maintained the Subspace Ansible system was completely absent. Later editions retconned the total destruction of the tech base in the Succession Wars to simply being crippling - there's still factories to produce Jumpships and mechs and the like, they're just very rare and heavily guarded. In-Universe, the situation also became better with the rediscovery of lost technology through the Helm Memory Core.
Battledroids also shipped with two alternate rule sets missing in later editions. "Expert" rules formed the basis of later editions rules."Advanced" rules removed piloting checks, fall damage, aimed shots and pushing attacks. "Basic" rules completely changed how the game played; rather than individual armor sections which could be damaged, mechs had fixed armor points, and would be destroyed once all armor was gone. Weapon systems could not be individual fired or aimed, instead all attacks were an Alpha Strike with one dice roll. Overheating, ammo expenditure, and physical attacks were absent.
Spell Cards were called Magic Cards, which was changed several years in to avoid copyright infringement of a certain other type of magic card. This resulted in a large-scale Retcon of an entire expansion name("Magic Ruler" was reprinted as "Spell Ruler").
Ritual Monsters, Counter-Traps, and Quick-Play Spell Cards were all not present in the starter decks or the first set. It was not until the above-mentioned "Spell Ruler" set that all of these card types were fully implemented into the game.
The ban-list was still not fully-enforced in the first few months, with certain cards not restricted or limited as they are now.
There was a greater emphasis on monsters without effects in the early years of the games, with Effect Monsters often being much less of a staple of decks.
Archetypes barely existed for quite some time, and the few that existed(Blue-Eyes White Dragon, Dark Magician, Harpie Lady) had nowhere near the type of support cards they often needed to really make them worth running, especially lack of an ability to easily Special Summon them. This resulted in a strong case of Complacent Gaming Syndrome when it came to deck design, as it often resulted in "beatdown" decks that involved lots of low-level, high ATK monsters and reliance on certain now-banned Spell/Trap cards. It was well into the GX era by the time Konami did a better job of creating viable archetypes and theme decks.
The art style for monsters shifted from a slightly darker, more Western-style appearance to much more manga-ish and zany. While Yu-Gi-Oh has always run on lots of Crazy Awesome and Ninja Pirate Zombie Robot in terms of monster design, severalmonsterslooked like they could have come out of a Dungeons & Dragons Monster Manual, while many newer monsters have a significantly more cartoony style.
Additionally, in the early days, there was some issue with templating. The original printing of Flute of Summoning Dragon was the first card to reference another card by name in its text, and it's the only one where that name is not enclosed in quotation marks. Flower Wolf had the word "FUSION" in it;s text box before listing it's materials. Finally, Polymerization said, "Fuse two or more fusion material monsters to form a new fusion monster," without actually explaining how you are supposed to do that. (The explanation was in the rulebook, since at the time Polymerization was the only way to get out any fusion monsters- thus, the card's effect needed to be explained alongside fusion monsters themselves.)