There’s a clear campaign against the mentally ill with the global rise of fascism. Lots of it shows up in anti homeless rhetoric, but you can see it in the MAHA and anti vaccination movements.
There’s no reason to use the word “r-tarded” to describe someone. As someone who’s worked with the intellectually challenged, it’s an insult to them to compare them with people who are willfully ignorant.
If a group of people are telling me this word was and continues to be used as a dehumanizing slur, that’s enough for me to look into a vocabulary change. More importantly, the very existence of a euphemism treadmill shows that you can’t stop at language change, and that disabled people need to be much more fully accepted in society.