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 situation you’re describing is a catch 22. Either people view it as ableist and it’s angering (though not necessarily insulting) that someone is using a slur, or people view it as a general purpose insult for stupid and it’s insulting. Plus, if someone uses a hateful tone and calls me something nice, it can still be hurtful, sarcasm exists, so even if people believe the word shouldn’t be used as an insult it’s not like that magically means it’s now a nice thing to do.
If someone thinks being overweight is t something to feel shameful about and is proud of their body while being overweight, it’s not like someone using fat as an insult magically no longer hurts or isn’t rude.