IPB and vBulletin had "threaded" mode where you would see a tree of replies, but it would only display a single reply, requiring you to manually select next reply to see in the UI.
In future I plan to track which post the poster clicked "reply" for and have UI to let you see direct replies to given posts, or see that post was made as a direct reply to other post, but I am not planning a "proper" tree view like how reddit does it.
Also some good opinion pieces from Discourse's creator on the subject:
https://blog.codinghorror.com/web-discussions-flat-by-design... https://blog.codinghorror.com/discussions-flat-or-threaded/
Reddit, Digg etc became popular partly _because_ of threading.
I think threading is the killer feature that allowed spaces like Reddit to so thoroughly replace "classic forums" over the last 13 years.