Honestly, the best way any opponent of free speech could convince any proponent of free speech is by giving them administrative power over a fractious, opinionated community whose members' well-beings are important to them, and seeing what sorts of norms and rules they develop.
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been there done that, I came out more pro-free-speech on the other side (caveat: we totally evolved rules that looked arbitrary from the outside, but they weren't that aligned with political correctness)
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our approach to harm reduction was not to stop people from saying mean things, but to enforce anonymity so that no matter what people said, it couldn't hurt them or others offline; people got banned for posting selfies but the politics overton window was way wider
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So yeah I'm not a proponent of total free speech inside communities (I know enough such communities and they're nasty) but imo communities should have near-total freedom wrt where they put their overton windows