And you just end up with poorly integrated moderation with extra steps when community starts cooperating to make it more efficient (e.g. maintaining filter lists).
Or there's no effective moderation so people that want more curated content and better UX moderation-wise will move elsewhere.
Nobody's forcing you to use moderated platforms either.
That said, I think the showdead setting in HN is good to have, so you can still opt to see content that would otherwise be filtered.
I think the GP was talking about usefulness of moderation in HN, Reddit, etc. in general. And parent was implying that filtering should exist only client-side (so no moderation by the platform), which I thought was unrealistic for some users that want moderation and who are then free to seek out more fitting platforms.
But yes, in a world where "moderation" is mandated by law, there'd be no alternatives.
That said, I think the showdead setting in HN is good to have, so you can still opt to see content that would otherwise be filtered.