Apart from niche forum who are still active but slowly dying, there’s not so much anymore, since social media came. Reddit was the right mix of forum and social media. Lemmy could be too. But it less easy to understand and use than a classical forum for common people. And there’s not this “organized” topic forum thing. (Reddit too).
But with social media become less comfortable to use, less interesting, perhaps forum alike will rise again.
Actually, if you just want to feel some “old” web, what it still active is “webrings” stuff. Basically, people make their own little website, with just html/CSS, and they are part of a webring, where you can surf between each website inside of it. And it tend to have this 2000’s web vibe.
Here’s one for example, but there’s lot a different webrings, with different theme sometimes. And it can be hard to find. https://ring.recurse.com/
Where ? It’s just a blank paper.