I smell Arknights
I personally wish they went with this:
Grimm are a very close relative to the Faunus species (animal features) so humans fear that the Faunus will turn on them and destroy everything just like the Grimm. Sort of like "Yeah, my friend wouldn't hurt a fly, but who's to say there isn't a little gene in him that will make him become evil?" The humans initially throw Faunus in cages and hide them away from civilization for their safety, but they soon think that they could destroy the "bad gene" in Faunus by consistently beating them, forcing them to work to their limits, starving them, and otherwise showing them that they are inferior and will never triumph over humans.
That's the simplest fucking thing they could of come up with. It wouldn't work at all for their racism "allegory" though since POC aren't a separate species from white people. If they wanted a realistic allegory, they should've just made everybody a Faunus and made a certain group be the victim of racism (ex. Dog)
That reminds me of Elder Scrolls V: Skyrim's Snow Elves/Ancient Falmer.
The Snow Elves were the very first race of Mer to exist in Skyrim up until the Atmorans (men) arrived. The Atmorans basically slaughtered and enslaved Snow Elves (they constructed Windhelm), and it got to a point where they had no choice but to try to seek refuge underground where their sibling race (the Dwemer) resided. Those who fled underground, "The Betrayed", were allowed to under the condition that they only consumed toxic fungi. Eventually, they became slaves to the Dwemer. They were even tortured. Through hundreds of years of devolution, there is only one Snow Elf left (Gelebor); any others are now Falmer, a race of blind, goblin-like creatures who are extremely isolated and aggressive.
Here's the full lore page here if you'd like a better explanation. TES has some godly lore.