Has anyone tried Twist ( https://twist.com/ )? It looks a lot like Zulip but with a nicer UI. I think it is crazy that Slack has not invested more in first-class threaded, context-aware conversations.
Beyond that, Zulip is open source and self-hostable, while Twist is not.
Side note, we have a chat about self-hosting set up on Zulip at https://homelabos.zulipchat.com/ with 85 members and growing. It's been great for new users to be able to come in and catch up on old discussions in a way that isn't possible with non-threaded platforms.