I can see how retard etc are offensive. It’s too resent. Like at school, spaz was a common insult as was spacca. I’d never use these words and it makes me even a bit uncomfortable typing them (my sister didn’t get the memo though and will still call use spaz but it an affectionate way like calling someone a dozy twat).
I just can’t see moron in the same way tbh.
On another note what about mong as in monging out on the sofa? It must come from mongol but it’s to describe someone a bit out of it just chilling instead of an insult. That’s still used pretty regularly.