"Good old" not in the sense that they are really literally running phpBB or Proboards, but in the sense that it's modern software and UI following the paradigms that once ruled web communities: Individual websites, with topics/threads in subforums, and mostly linear reply chains[1].
And it works. Information seems much less ephemeral, and I can check in to the forum at my leisure, read through old and new answers to what I find interesting, without a sense of having to be in the moment at "real time". There is also no or much less of any algorithm that sorts out the content.
I guess people must have gotten tired of this current state of fleeting, low-information-bite chatting on Discord and Twitter on one side, and the disjointed immaterial seeming mess that Facebook Groups, which I think replaced a lot of old forums, was. Even reddit and HN feed into this ephemerality with their algorithms that age out older topics and the tree-like reply structure: It's very good for news, but not good for topics lasting beyond the freshness of any news.
That being said, the Discord approach seems wrong, unless the forums are also accessible via browser. I personally want it to be a website, being able to link to posts and making it part of my daily trawl of websites.
[1] Some link or allow to uncollapse replies to other replies while also keeping every reply linearly in the thread.