The "right to not be insulted" is literally censorship.
Prohibiting slurs is like prohibiting nudity in public parks. You're prohibiting an action so that people don't self-prohibit themselves from visiting.
Yeah, I know… everybody is a winner and what not.
First they banned murder, then it was punching, next it was looking.
The problem with slippery slope arguments is that you need a slope.
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The thing you're missing is that it's really a "you" problem. Most people don't go on the internet to read slurs just like they don't go to public parks to look at penises. You want to post slurs and so you want to remove the rules against it. The result is just going to be people not visiting 4chan.
Censorship is suppression.
Insulting someone or discriminating against them is not supression, it's unlawful.
If that's not your cup of tea, I'd challenge you to forego any and all (!) ameneties of modern civilized society to enjoy your anarcho-freedom.
Don't pretend to be a tough one by sitting in a comfy chair and picking the raisins out of the dough.
It is not, at all, unlawful to insult someone. Your argument is absurd.