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.