I'd expect a truely decentralized forum to work without website. Just a program you run, maybe like Tor browser, with a local database (or blockchain) containing all posts ever created. If most of the users use "light clients", I wouldn't call it truely decentralized.
Well, you can't dictate your users which type of client they should use 
If I imagine a decentralized forum client, it doesn't have to be restricted to only one forum: many different forums could be accessed from the same client, and every user could create their own forum.
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The crucial question for me is that it should be possible to use it without any restriction directly connecting to the blockchain (or Torrent-shared database), and ideally there should of course be an easy to use interface to post this way. If anybody however wants to run a web frontend publishing the posts from the blockchain there, then nobody can stop them
So, hypothetically, the forum could be accessed through a browser through one of many centralized servers (which together provide decentralized access), just like you "access" Electrum servers through the Electrum client. That sounds good to me!Quote
Basically the web frontend operators would compete for the top SEO spots but they would harm the whole project if they compete too hard. If they don't work commercially, they could however refer to one version of the web-delivered content as the "canonical" one.
If many individual "points of entry" are competing for SEO, users can just choose the one that doesn't add too many ads. Or the fastest server with the best uptime. I hope to see this in reality some day.