nodeBB literally is forum software in the Fediverse. I don't know if it can do strictly linear like phpBB, though.

Or you create a channel on Hubzilla. You can configure it for your stream to be strictly linear as opposed to be tree-style. And then you can join Lemmy communities, /kbin or Mbin magazines, PieFed communities, nodeBB subforums, Friendica groups, Hubzilla forums, (streams) groups, Forte groups, you name them, and they're all strictly linear from your POV. It's just that you, on Hubzilla, will always only directly reply to the start post (as opposed to the most recent comment) when the thread itself at its source is actually tree-style.