All insults are forbidden in civilized conversation. Were you not aware of that? :dubious:
As for which insults should be legally forbidden in a free society, the answer is pretty much none, with possible “fighting words” exceptions of the sort that constitutional law has already sussed out.
For the large grey area between the rigorous scruples of civilized discourse and the (almost)-anything-goes approach of legal protections of free speech, variously constituted authorities (e.g., schools, businesses, etc.) may make their own rules about what they allow people to say under their supervision.
If some of those various rules tend too far to one extreme or the other in somebody’s opinion, that somebody is free to complain about them. Politely, of course.