In the now-defunct official Games Workshop webboard, posting anything about the Squats would typicaly result in the thread being deleted, and the thread-starter banhammered.
Predictably, a segment of the fanbase has decided that Squats and Zoats were the bestest thing since sliced squig, and mourn their disappearance as further evidence of GW having lost their soul.
Games Workshop has tried to covertly resurrect the Squats in the form of the Demiurg, a race of Space Dwarves under the Tau Empire.
Recently, the Zoats have returned in Warhammer's Storm of Magic suplement, as summonable monster allies.
Warhammer Fantasy also has the Firmir, a race of Cyclopean monsters that were essentially entirely excised from the fluff and had their army discontinued. Much of this might have to do with the questionable way they reproduce. Recently they've gotten little minor references in the rulebook, a summoned monster in Storm of Magic and a few Forge World models, but a new book is highly unlikely. Chaos Dwarves also seemed to be going that way, but they recently started get large amounts of reference in the fluff, mostly due to their proximity to the Ogre Kingdoms and their popularity with older players. Every time there's even a hint of something new coming, everyone will declare it's the Chaos Dwarves.
Later editions of Vampire: the Masquerade did their damnedest to sweep everything from the Dirty Secrets of the Black Hand supplement under the carpet.
Gary Gygax has said that he regretted a number of rules that he felt pressured to put in various versions of Dungeons And Dragons, singling out psionics, the monk class and weapon speeds and effects versus armor as egregious examples.
The sexism of 1st Edition AD&D is likewise something which later editions' designers would very much like everyone to forget.
Urza's Saga block, which was massively overpowered and created the most unfun standard environment in history, according to Mark Rosewater the only block where "The entire team got called into the boss's office and got yelled at." To a lesser extent, any other overly format-dominating cards/archetypes.
On the other end of the spectrum there are Homelands, Fallen Empires, and The Dark, widely considered the weakest sets. The game, at the time, had been having problems with overly powerful cards, and had corrected too far in the opposite direction. Fallen Empires also has the distinction of being overprinted as well, making cards and packs next to worseless.
Exalted has the much-reviled Scroll of the Monk, for writer Dean Shomshak. The first thing he did upon becoming an Ink Monkey was apologize for writing it.
