That's probably why there's not a single decentralized forum yet. And without centralized server, everyone will need to keep the entire database of all posts. It won't run in a browser, so no advertising for SEO either. Search engines don't visit decentralized forums.
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.
However, I wouldn't say that steemit is censorship resistant.
Although the backend has some decentralization, the front end (which controls all the traffic,i.e, the value) can easily censor words, authors, etc... and the founders can be coerced.