My institution turned on the Beta version of discussions and consequently all threaded discussions vanished with no way to re-enable. I reset to the legacy and the ability to thread returned. This has to be a bug -- unless there is a workaround with instructions to be published. But then I am seeing poor design practices everywhere theses days -- please escalate -- this isn't a NEW idea.
Focused Discussion (default setting) enable a single response and replies to that response. A focused discussion is fairly easy to follow;however, it can be difficult for students to show who they are responding to in the discussion.
Threaded Discussions enable infinite nesting or threading of comments, so that students can branch off into various side discussions. This also makes it easier to see who the students are respond to and communicating with in the discussion. The threads can be open and closed for easier viewing as well.