Warhammer has races that silently disappear between editions. What happened to the Fimir?
They made a recent appearance in the Graham McNeill Sigmar-era novel Empire. The 'mist daemons' are never definitively labelled as Fimir but it is very clear what they are supposed to be.
The Chaos Dwarfs in the Warhammer Fantasy world have also simply disappeared. To the point that even though they are still included in the last official Blood Bowl rulebook, they are the only official race not in the computer game.
They were mentioned, repeatedly, in the second edition of the role-playing game.
They are still mentioned quite a bit, both as a source of equipment for the other chaos factions and Ogres and as the origin story of the Black Orcs.
It seems likely, in-universe anyway, that there's simply too few Chaos Dwarfs to make a full army out of them, even with Hobgoblin and Orc slaves supplementing them, as the scarcity of their race was something mentioned repeatedly.
Chaos Dwarfs have seen a resurgence as of late 2011, given that Forge World has started producing a line of them in Warhammer Forge. Whether or not they'll return to being a mainstream army is yet to be seen.
Bull Centaurs have it even worse. Chaos Dwarfs have three models being consistently produced as Hellcannon crew (admittedly without the stylish headgear of the classic Chaos Dwarf range). Bull Centaurs have vanished entirely.
Malal, the fifth Chaos god, was dropped around the 3rd edition of Warhammer Fantasy. This happened because the people who held the Copyright left Games Workshop. An entity called Malice, a Suspiciously Similar Substitute, has since appeared in Warhammer 40000, however. In addition, Be'lakor the Daemon Prince in Warhammer Fantasy share's many attributes with Malal.
In Warhammer 40,000 the Squats vanished as well, partly for low sales but mostly because they were too silly/campy in a setting that was becoming Darker and Edgier. Word of God has usually been "the Tyranids ate them all" (supported by the "Abhuman" section in the 6th edition rulebook) but they have occasionally tried to claim "they never existed". Blame Chaos, perhaps.
The same happened earlier on to several models in the 40k range with the simplification that happened in the 3rd and 4th editions. However some of them have been making a comeback in recent editions, most notably Bjorn the Fell-handed and the infamous Jokaero.
Another prominent example are the Illuminati (possessed psykers who manage to overcome the Daemon that possessed them, eventually recycled as the Exorcists chapter of Space Marines), the Star Child (a Warp entity that supposedly represents the Emperor's soul, today laughed off as a delusion of the Eldar) and the Sensei (supposedly the "sons" or descendants of the Emperor).
Despite being featured in a couple of cards when the Gravekeeper theme first kicked off in the Yu-Gi-Oh! TCG, Cobraman Sakuzy and any references to him completely vanished in later expansions, most likely due to not being useful at all in any deck and being an ill-fit for the theme.