Sure. Just look at Wordpress… it’s a blogging platform rather than a forum, but it has an ActivityPub plugin available that allows federation of blog posts and comments. ActivityPub is a standard published by the W3C (the same organization that oversees the HTML standard, among many others). Anyone can implement the standard in their software if they want to.

You’d be asking PhpBB or Discourse to implement ActivityPub. The latter already has a plugin for it, but PhpBB doesn’t have much discussion on the topic

Yep, pretty much anything can implement it. Discourse is working on an ActivityPub plugin.

Don’t be too modest, what good forums are you talking about? 👀

In the meantime, https://github.com/LemmyNet/lemmyBB exists.

NodeBB is also working on activitypub integration

Sure. Reddit could, if they wanted, implement activitypub and join Lemmy, and Twitter could too and join Mastodon. Of course, since their server software wasn’t designed to do this it would be a significant investment and they won’t want to do it for a slew of business reasons, but if they really wanted they could.