Language evolves. “Mental removedation” is not longer recognized as as medical term, its use in modern times is generally regarded as a slur by the disability community and is treated as such in pretty much all communities that actively try to include neurodiverse people are oppose ableism.
So to answer the question of “ince wshen”. The answer is “for at least a couple decades now”. I was actually hit by my friend with a severely disabled brother for using it back in like 2006.
I am active in the disability rights movement and pretty much all of us that are agree the word is a slur and there is no excuse to be using it. Its not a “word for mental capacity” anymore and hasn’t been in a long time. Plus, insinuating that mental disability is a BAD thing is ableist on its face.