In contemporary language, that word (among others) is almost entirely used as an insult by way of equating somebody's intelligence with those who have intellectual disabilities, which creates a negative connotation. Similarly, this is why we don't say things we dislike are "gay" anymore. It's disrespectful to the people who actually fall under the definition, and it proliferates negative associations with traits that people are stuck living with and had no choice in acquiring.

The only reason "idiot" hasn't followed suit is because it's much more culturally ingrained, and there's hasn't been as significant of an attempt to change it as with other words.

I've never seen anyone use or interpret the r-word as a slur outside of lemmy

It's not exclusive to Lemmy, but it is mostly left-leaning spaces or gen Z individuals who see it that way. Center and right-leaning spaces see treating the word as a slur to be censorship (as opposed to being respectful of others) and keep using it or actively push back by saying it more.

Not Lemmy specific. There was US legislation related to the word being deemed offensive fifteen years ago (given the slow nature of Congress, it wasn't a new sentiment then, either): https://en.m.wikipedia.org/wiki/Rosa%27s_Law

Fair enough that plenty of insulting words could be cast as abelist: but my guess is that a word like "idiot" is old enough that most folks called that in a medical context aren't around any more. Maybe I'm wrong though: plenty of folks do push against saying things like "crazy" in an insulting manner.

You must not be close to anyone that has any sort of mental illness. It's a very broad and hurtful term. It has been for a long time.

Edit. Found this online

https://www.spreadtheword.global/resource-archive/r-word-effects