I have. Not very often, but it has happened at some points.

I think using retarded to mean stupid could be part of the problem, actually. Like your Elon example, tbh. The fact that it is being used in a way that refers to mental faculties could cause offense to people with delayed mental faculties. Using the word to mean "slowed", in the original meaning, without referring to mental faculties, is probably more likely to rehabilitate the word apart from the use as a slur.