There's kind of a treadmill of linguistic acceptability going where the technical term for something slowly, by misuse, becomes a slur. The R word is the latest of these, and I think the best comparison for it is the word de-generate (verb, not noun). They're both t e c h n i c a l l y scientific terms, one means to slow and the other means to decay or worsen. But in the context of early 20th century US medicine, when abuse, lobotomization and even sterilization were go-to treatments for all kinds of mental illness, such words became weapons used by the medical establishment to dehumanize "undesirables" and justify their forced "correction". Many such words in the field have run this course: originally intended as neutral descriptors, but their repeated invoking for fascist purposes leaves a stink of nazi on them.

Then there's the whole trickle down cultural aspect of the R word, everyone using it casually for decades (my partner is watching Dexter for the first time and they drop like 3 per episode, it was unavoidable in the US from like 1980 to 2015). But that's more broad, and I'm sure people have written it up already. I just wanted to try and shed some light on the word's more sinister historical use by those with actual power. If you're a G*mer, the word is how you express acute dissatisfaction. But if you're an unempathetic doctor in a capitalist regime, especially back in the day, it's the word you can use to ruin someone's life.