Here's what I think: say literally whatever you want. Free speech is important. Understand there are consequences for your speech; they may be minor (someone's feelings are hurt), they may be major (you lose your job, you get punched in the face). Policing speech, though, is categorically bad.
I try to consider the POV of the person I'm speaking with as a courtesy I enjoy when others show it. I know even that mindset is hard for people, especially with seemingly new courtesies like gender pronouns or other signifiers. And I recognize that some people wholeheartedly reject the concept that the words you choose are important; having thick skin is an important part of being an adult too, and you won't make it anywhere in this world without it. I think there's a difference between using a slur on a message board and using it in real life too. I have a sibling who is not heterosexual. If someone called her a slur, I'd probably take issue with it. See above about major consequences.
The reason I choose to weigh in on Knight is not because I find what he has to say offensive. It's because I think he has a poorly developed sense of humor. Whereas by comparison I think Milhouse is a funny dude. Would I use a slur like that? No, but I don't really get offended on behalf of others either. Tricky stuff, above my paygrade, but my $0.02.