BattleTech has a meta-example. When designing many the game's earliest Humongous Mechas in 1984, the FASA Corporation contracted outside art companies or simply licensed the designs from existing mecha shows, primarily Macross. Long story short of a messy lawsuit due to licensing confusion in 1994, FASA was no longer able to use artwork - either original or licensed - of these mechs. While these "unseen" mechs still existed In-Universe, they could not be depicted in artwork or produced as miniatures for the wargame; all publications with artwork of these mechs had to be taken out of print and edited. After a number of clumsy attempts to get around this, the designs were completely replaced/reworked (via Retcon) in 2015.
In-Universe, this was the treatment given to the Not-Named-Clan, i.e. Clan Wolverine. When it became obvious to Nicholai Kerensky that the Wolverines were unwilling to play ball with the new society he had envisioned, in favour of simply doing things as efficiently as possible to increase their power, the Wolverines found themselves slowly pushed out of Clan society, framed with a False Flag Operation, and subsequently Annihilated. Kerensky then banned any mention of the Wolverines, to the extent that even comparing a clanner to the animal or forcing them to use a WVR-6R "Wolverine" Battlemech is considered a dire insult. Another apparent victim of this is a Clan Battlemech called the Hellhound. A number of (now)second-line battlemechs were improved versions of rather classic designs(all of whom happened to be part of the "Unseen" in the meta-example) with the moniker "IIC" added to them. The Hellhound seems to be the sole exception of these, but it's size, design, and armaments indicate that it would've been a dead ringer to be called Wolverine IIC.
Iron Crown Enterprise's Cyberspace cyberpunk RPG. Blanks are people who have either never had a file on them (born without being documentated) or have had their file deleted from all databases. They don't have to pay taxes and are hard to track down, but if caught they are subjected to "thought reorientation" (brainwashing/mindwiping).
If an elf in The Dark Eye does something really bad, their clan will weave a spell to forget they ever existed and become unable to recognise each other.
In the Nentir Vale setting, Asmodeus erased the name of the god that he rebelled against from history. Very few people know the god existed, those who do know him only as He Who Was. This is because Asmodeus feared what would happen if someone said the slain god's name even once.
People who are made God-Blooded by Vecna, god of secrets, have all knowledge of their past life erased from record and memory; only Vecna and the God-Blooded remember who they were beforehand.
In Exalted some advanced Sidereal charms are capable of killing someone so thoroughly that they were never actually born.
The "Zeroed" advantage in GURPS, available to secret government operatives, The Men in Black and the like.
Magic: The Gathering: In general, exile effects are often flavoured as the total removal of something from existence, but so go further and also involve their removal from history, memory or at least records.
Revoke Existence, which does this to artifacts (machinery) and enchantments (long-lasting effects). Even the flavor text fits this.
—"No half measures, no regrets. We'll tell no stories of this day. It will be as if it never existed at all."
—Ganedor, loxodon mystic
Door to Nothingness reads "All memory of your existence will be wiped from reality. You will die, and no one will mourn."
The flavour text on Cast Down: "Your life is finished, your name lost, and your work forgotten. It is as though Mazeura never existed."
Alluding to this trope, when the Fifth Edition was thrown to Canon Discontinuity, it was officially declared an un-product.
In the second edition's post-MegaWhoops period, there were still information terminals hooked up to The Computer's old personnel database, but due to data corruption, a few clones weren't recognized: the terminals would glitch and cough up random unrelated data instead, both a blessing and a curse.
The XP edition supplement The Underplex had DELETED citizens. All records of them were wiped from computer data banks (including those of the Computer itself) as a result of error, revenge, system failure or bureaucratic idiocy. It also happens to Troubleshooters who are out of contact for too long.
Pathfinder: Irori, who is among other things the god of history, can enforce a metaphysical version of this on mortals who sufficiently raise his ire by editing the memories of everyone even tangentially connected to the target to make everyone forget that they ever existed. If he's angered further, he extends this to causing written and pictorial records to be edited to omit any mention of or information on the luckless mortal as well.
Pokémon Trading Card Game: This ends up being Kadabra's fate after Skyridge due to a lawsuit from Uri Geller. There hasn't been a Kadabra card since, with this sometimes extending to Abra and Alakazam as well. The most egregious example of this is in Mysterious Treasures, which has Abra and Alakazam, but skips over Kadabra. That set's Abra even has an attack that lets you search your deck for Alakazam and put it onto Abra, thus negating the need for a Kadabra card altogether. Uri Geller eventually waived the ban in November 2020.
For anyone wondering, the original Japanese names of these Pokemon were all references to well-known psychics/magicians with Kadabra's Japanese name being Yungerer which is meant to sound like Uri Geller when spoken outloud.
In Shadowrun people without a System Identification Number (SIN) cannot legally make purchases, sign business contracts, or vote, they're not even considered citizens. Lacking a SIN also makes it much harder to track one's criminal activity so the majority of Shadowrunners are SINless but also use one or more fake SINs. Editions that use character advantages list SINner as a disadvantage, and there's a SINless advantage where one not only lacks a legit SIN, but any attempt to register a legal one for them (like when a SINless person is convicted of a crime) is erased by mysterious forces.
Last Unicorn Games' Star Trek: The Expanded Universe supplement All Our Yesterdays: The Time Travel Sourcebook has the Disadvantage "No Past". It's usually caused by either a character coming from outside of civilization or all of their records being deleted. It could also be caused by a time traveler altering the timeline and erasing the character's personal history or if the character is from an alternate timeline. The character is difficult to track down or identify and is thus a valued espionage operative. They also have all of the disadvantages of not having their existence acknowledged: can't own property, doesn't have any rights, etc.
Vampire: The Requiem: Back during the days of Rome, it turned out a clan was suspected of working with the strix. The clan was wiped out down to a man, and all references to their name stricken — they are only known as the Traditores (or "traitors").
Often attempted to varying degrees of success in Warhammer 40,000, when the Inquisition or other Imperial agencies try to sweep some information under the rug. Nobody's sure what happened to the 2nd and 11th of the original Space Marine Legions, only that all records of them have been scrubbed, "order designation unknown," and likewise the Space Marine chapters of the so-called "Cursed Founding" got this treatment after breaking out in severe mutations, only handfuls surviving as traitors or extremely closely watched loyalists (with one, the Minotaurs, apparently being used to legitimize an ancient unit of loyalists from the Iron Warriors Traitor Legion). The Imperium has tried to erase all references to the Horus Heresy to hide the fact that Space Marines can fall to Chaos from citizens, but depending on the planet Horus may be completely unknown or commonly referenced as a bogeyman (as a side note, whatever the 2nd and 11th Primarchs did it was bad enough that even Horus held to the Edict of Obliteration after his betrayal). This is either due to GW's relatively low standards onCanon, or because it's hard to eliminate every trace of someone from countless sprawling data-crypts spanning the Imperium.
Games Workshop's actions regarding the Squats are eerily similar to this trope. For context, the Squats were effectively space dwarfs that were originally introduced in 1987 in Rogue Trader, and there were plans to have their own army book back in the second edition of the tabletop game. The earlier developers of Squat lore left the game a few months before the transition to third edition and the remaining crew simply didn't have the artistic desire to make them, quietly removing mentions in the last few second edition codices released in 1997 (last year of second edition) before scrubbing them entirely from the third edition rulebook. It was stated by Jervis Johnson that they simply didn't have the creative desire to build a Space Dwarf army as they felt that they couldn't do the Dwarf archetype justice like in Warhammer Fantasy and had became averse to marketing 40k as WFB in the future as it interfered with WFB's storyline of The World Is Always Doomed and placed 40k in our far future while implying they shared the Warp and Chaos Gods to a degree and the Old Ones of Fantasy and 40k were one and the same, with their disappearing from Fantasy a result of the first awakening of the Daemons in 40k's War in Heaven forcing them to abandon their project on Fantasy's world. With the original designers long gone from the company, they made the Demiurg as the Space Dwarf race and mentioned them as selling their services to the Tau and included them Battlefleet Gothic. The official stance of Games Workshop shortly after that point was that the Squats never existed, and to reinforce this the publishers removed all mention of the Squats from publications and ceased distributing any works where the Squats played a significant role. When the GW website was active, so much as mentioning the Squats would get you automatically banned and the thread you posted in deleted, though the forums were defunct not long after this policy was instated. If anyone asked questions about Squats at GW-sanctioned events, they'd have security remove the questioner from the event. Although Jervis would encourage in the same statement that the old models could still be used to count as Imperial Guards, GW really, really did not like the Squats. It was nearly two decades before things finally relaxed, and the writers were allowed to give Squats an in-canon send-off (eaten by Tyranids). They were even referenced in the 6th Edition Rulebook. In 8th Edition, Squats were confirmed to still be alive, but at greatly reduced numbers. Grendl Grendlsen , a mercenary, was released for Gaiden GameNecromunda. After the WarhammerTheEnd Times campaign destroyed Fantasy's world (revealed to have been called Mallus), successor setting WarhammerAgeofSigmar has the Kharadron Overlords, a Duardin (successor race to the classic Dwarfs) faction that uses airships like the Squats, prompting some players to customize their models to be like Squats.
The Squats returned as a main game faction in April 2022 with two trailers, the first of which was an April Fools double bluff.
The Soul Drinkers chapter was obliterated from all historical records after they were judged to be heretics and traitors, an action that actually hindered the efforts of those seeking to bring them to justice, because they now had no records to work from. In the end, this was revoked and those members who helped repel a Chaos assault led by Soul Drinkers who genuinely had fallen to Chaos were posthumously restored to the records. Ironically said assault occurred during the trial of the Chapter. Only three loyalists and one of the traitor leaders survived, with the loyalist survivors going into exile in the Warp dragging the remaining traitor with them. The traitor, at the tender of lack of mercies of the Chaos Gods, is almost certainly dead and the last loyalists were marked as missing in action. Further, the chapter was rebuilt with Primaris Marines in the Ultima Founding.
" mechs still existed In-Universe, they could not be depicted in artwork or produced as miniatures for the wargame; all publications with artwork of these mechs had to be taken out of print and edited. After a number of clumsy attempts to get around this, the designs were completely replaced/reworked (via Retcon) in 2015.