Here we have to distinguish between the platform and the client.

If somebody wants to found a Bitcointalk-style decentralized forum, create a (e.g. Steemit-like) blockchain, and set some rules for it, that could be achieved with a dedicated "platform". Another idea is to use an existing "platform" like the various decentralized Reddit clones and create a Bitcointalk-style variant of it. Or, even alternatively, you could design a client software (either local or web-based) which is able to access several "platforms" (e.g. different Mastodon, Reddit-clone etc. instances), filters them for Bitcoin-themed "subforums", and creates something like a local Bitcoin-themed forum coming from different platforms.

Every possible variant of these concepts is possible. There is however one main problem one has to deal with: spam. For example, let's say you create a decentralized forum where everybody could create a subforum. How do you prevent that subforums are massively created?

Basically this is the same problem than the problem with moderation. There are two approaches to this: one is that you could implement a governance function, e.g. with a token. This was basically like Steemit did it: if a post received too many downvotes it wasn't displayed to users by default, and this could also happen to subforums. You could also the users require to vote for subforum creation, but due to sybil attacks you would probably need a PoS-style system, which has also disadvantages.

But there is another approach: let the clients (e.g. the web interface operators, or the local software) decide. These softwares could include an own content classification and also moderation system. This could also be a factor for them to compete: who has the most interesting and attractive interface?

I think you're defining "forum" too narrowly ("websites that look like the forums of the 90s and 00s, like Bitcointalk "). Twitter, Instagram, even TikTok are also "forums", only with a slightly different interface. These are platforms allowing people to discuss, react to other posts, and categorize posts thematically (if that's done via folksonomies - e.g. hashtags - or ontologies - subforums defined in some kind of a "top-down" manner - isn't that important.) There are even some successful forums which are very close to the 90s instances, particularly Reddit and their clones.

Of course instead we could talk about a decentralized Bitcoin-focused social media platform. There are many functions in modern social networks I would like to see in Bitcointalk, for example to follow other users and be alerted in the watchlist whenever they post. This is absolutely no problem in a forum software, modern softwares like Discourse allow it. And this would also be no problem in a decentralized forum.