They were never removed, it's just that, since the uniques creatures introduction in the game, they became a bit rarer
I honestly think unique creatures were a mistake, and should have been dungeon or thematic mini-biome specific. Or at the very least, very rare and random (like one single unique creature per main biome, and completely random and not biome-specific. IE instead of always having hordes of Gleaps, Nutmiges, bulbops, and poptops on garden worlds, You might see 1 family of Fennix OR one group of Gleaps OR one or two Nutmiges, and everything else is ProcGen. Bring back the procgen monsters with the barf-rainbows and the not-cliffracers shooting dead fish all over the place.