I never really understood the difference between insults and slurs. Somebody said that slurs target groups of people and imply that just being in the group is bad. That explanation works for ethnic/racial slurs or the ones regarding gender/sexualities.

But to me “retarded” is not specifically tied to a group. It’s not mainly used for people who are disabled or neurodivergent for example. As far as I’m concerned it’s the same as calling someone a moron or incredibly stupid.