You know who was never ever called r-??? The kids with learning disabilities.

The x key on my keyboard just flew off into the sun, I pressed it so hard.

How would you feel if something you couldn't change about yourself was used by your peers as an insult? You really don't see how using the word "retarded" as a stand-in for "stupid" is still an insult to people with mental disabilities? If someone was acting stupid and you said they were acting "like a Chinese person" that's still insulting to Chinese people even if the person you're targeting isn't Chinese.

Also I'm calling shenanigans on "the 1980s definition". I am the same age as you and I was taught it was cruel to use the word that way back then.