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.
The fact that it’s still currently used as a euphemism for people with actual conditions like down syndrome is precisely why it’s problematic to use it as an insult. Previous words used in decades past are only insults now and no longer used as a euphemism for the condition.