Zulip's threading model is much more like a lightweight version of email threading -- when you start a new thread, you pick a short topic for it, and then after that people just reply to the thread.
Not exactly the same, but you may find a product I worked on a few years back, www.kona.com, to be of academic interest as someone who is looking at different communication methods.
We used a similar "topic" method and allowed users to fork conversations to have the best of both worlds -- linear chat for most cases and topical chat when a breakout was needed.
