Nah, no ofence to developmentally disabled dudes intended. As for the use of the word, I'd say it's mostly due to my personal deeply negative outlook on language cleansings and some cultural differences. The former is about feds trying to enforce what we can and cannot say, so it's not terribly relevant in this case, but the latter is more interesting: for example, in russian we have words "педераст"*/"пидор", which without context are more or less equivalent to english "faggot"/"fag", and would indeed be considered derogatory when applied to gay fellas; however, most often they're used to either jokingly or not call sb a bad person and don't imply anything about their sexual preferences. Even closer examples are "даун" (a short for a person with the Down syndrome, which is a currently used diagnosis, but hella rude when applied to them; use the full phrase if you ever want to speak about the disorder in russian) and "олигофрен" (pretty much "the r-word", as you call it; also an old name for a medical diagnosis; quite rarely used nowadays since it doesn't really roll of the tongue, but is actually somewhat acceptable in public if you want to call sb. out for their monumental stupidity). So, to me it's quite unusual to somehow associate ppl currently living with a disorder with their historic negative image attached to a word that is no longer commonly used to refer to them; ppl live on their own, and so does that bunch of characteristics they back in the stone age were supposed to possess.

*technically, it's derived from greek, and means "pedophile", and was even used this way, I think, in the late USSR. Don't really remember exactly, since it's been a while since I went into that etymological rabbithole. So, yeah, even the image changes from time to time