"Retarded" can't be a slur because it can be used to describe slowed/inhibited things that aren't people. "Retard" is a slur derived from the adjective "Retarded". Unlike the F-slur, N-word, and all the other colorful terms hateful people use to show people that they aren't welcome on the basis of their identity, retarded has OTHER MEANINGS, and it is so much more apt a word than "Dumb" or "Slow" in so many contexts that it's frankly (choose your adjective here) that we should have to walk on eggshells around it.

Expressing disrespect for a person for things outside their control is cowardly and close-minded. We should censor people who try to co-opt the group they are speaking in to express their prejudice. But extending the censorship of a slur to its root word, even for innocuous contexts, is an overreach of the social policing of our language. It sets a bad example, since ANY WORD can be made to be an insult to someone if used that way, and we set a bad cultural precedent by doing this for "retarded"

I understand that there's no council that decides what is or isn't acceptable to say, but I really wish people would think about this with a little more nuance than just "R-word detected, speaker shall be shunned" without considering the context. The way I see it, refusal to consider context is a redirection of the same kind of prejudiced thinking that makes slurs bad. But it's being applied to a person's speech rather than their identity, so it's not as bad a thing to do.