The closest thing I saw to a decentralized forum that worked was Steem(it). It ran quite well (it still exists) and had good SEO for some time. The crucial part making that possible was obviously that there was a web front-end run by the founders, but the content was stored in a decentralized manner. So there were a couple of alternative web front-ends.

(OK, it is not really a forum, more a kind of blog platform. But a forum and a Steemit-style platform is esentially the same thing, only with different CSS style )

Let's say such a kind of forum is implemented, and one server is taken down. The community could still post on other servers in friendlier jurisdictions, or directly to the blockchain. The web server operators could also decide if they moderate some kind of content, for example illegal stuff. In theory each web frontend operator could have its own moderation.

Steem(it) had of course the problem that it had centralization problems because the governance token was highly concentrated on the founders. And of course the monetization system had several flaws.
 ut actually such a system could be designed with varied grades of (de)centralization. I also personally think that the on-chain storage was not really well done; in the way they did it it could not really scale as others already wrote. I'd better store only the hashes of the posts on-chain and share the complete content via a torrent-style system. This would also make it possible to make illegal stuff really disappear.
But I think Steem(it) showed the principle of offering a decentralized publishing platform with web front-ends can work.

So in general I'm not so pessimistic about the "decentralized forum" idea.