We already have illegal speech, and this slippery slope you’re so afraid of hasn’t appeared. The boundaries seems pretty well contained by social convention and contract. Stopping people from saying the n word online isn’t going to lead to the fall of democracy. In the upside down hypothetical future where we decide as a society that “akshually racism is good”, nobody will be getting arrested for saying the n word by then. Don’t worry so much about far future hypotheticals, worry more about concrete harms being caused today.