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
Is Bitcoin less decentralized because users use Sparrow/Electrum/mobile clients?
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; and this additionally would make SEO possible. And this was true with Steem too, as all posts were stored on the blockchain. However I'm not sure in this case how user-friendly the Steem blockchain client was for users wanting to post and read.
A web front end existing would not make the project more centralized, if it is always clear that the originating posts come from blockchain/torrent/IPFS/ZeroNet.
The only possible issues I see with this "hybrid model" are copyright and duplicate content issues. Copyright can be relatively easily solved, you could simply require a Creative Commons license from all users posting on the forum. But duplicate content depends on the search engine algorithm (e.g. Google's). 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.
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.
The problem of Steem(it) was not the decentralization of the content. It was that the founders-run web interface (which was not the only one) was too dominant. People talked more about "Steemit" (the main web interface) than about "Steem" (the blockchain project), and that was for sure also intended, because so it was easier to promote. There was also the issue of the "sponsored accounts". This was a requirement because on Steem's blockchain there was a fee to create new addresses, which was quite high for some time when Steem bubbled (more than $5), and only if you registered on the Steemit web interface you got an account for free.
But all these are flaws that don't have to be repeated by a really decentralized project. The crucial issue is that the database distribution and content creation are decentralized. What the web frontends do is their business (as long as the copyright issue is solved, see above). If one of them is coerced to censor information, users can switch to another one.
Main problem with Steem was tokens they used to reward members for writing posts and comments, and I think this project later forked into separate project with different token.
Only way for forum to work in decentralized way long term is if there are no tokens involved, and you should not have to pay to make posts.
In my opinion, a token could be involved if it's necessary to secure the blockchain and it's distributed in a decentralized way (e.g. via PoW). Even if it's a bitcoin sidechain, it could be still necessary to run a different consensus algorithm to secure that sidechain (if it's not merged-mined). (Such a possibility is for example one of the reasons I'm so interested in sidechains
)
I thought however that Steem's monetization model wasn't bad, it was basically similar to what projects like "
Flattr" intended to achieve 10 years ago. What however was totally bad in Steem was the initial token distribution. It seems there was no premine but the coin was heavily instamined by the founders in its initial stage, when it was promoted as an "experimental blockchain" without any marketing and so there were few miners interested it. Later they switched to DPoS.