Let's say someone come up with a decentralized forum software where no authority can touch. That's pretty much what darkweb is. Every type of filth will leak from a place like that.
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.
Theymos wrote about the possibility of a decentralized moderation system, kinda like the current Trust system. So if you trust me, you won't see posts I've deleted. But if someone creates a billion spam accounts, that's still going to be a problem.
I expect to be stuck with a centralized forum for a very long time.
the best you could do is a three or four server mirror system.
all in different countries.
but this still means individuals attached to one of the servers could get grabbed up and charged by a country.
We have to face the simple fact that the world is not as free as we would like.