It's always been a condition, and medical professionals still use the term MR as shorthand for mentally regular to describe their client's condition. There are varying degrees of MR, and if you don't think using that word hurts people, you should really ask around. My lil bro works at a house with clients how have varying levels of MR, and they overhear people insult them. You'd be fucking wrong if you think they don't understand the connotation and are hurt by strangers cruelty.
I don't want to come off preachy because I used to use that word as an insult as well. I knocked that shit off, and you can too, it ain't hard.