The 90s: I used to use it back in the day, until one time I volunteered for a handicapped kids event, and saw how this one kid with Downs got really upset at someone calling him “the ‘R’ word” (I had never heard it put that way, or even considered it as truly harmful).

Anyway, I wiped it from my vocabulary after that. I still consider it a slur, unlike idiot, which is more generic, and not used as a specific slur against intellectually disabled folks.