The only other type of thread design I know of is Reddit-style threads in a chat app. The negative of that is that the threads cannot be directly incorporated into chat itself after creation because they become things of their own, just like Reddit threads, but the positive is massive: threads are entities of their own and they can be infinitely nested and referred to indefinitely into the future.

An example of this design is Aether. (Disclosure: I work on this, aether.app) If you have issues with Slack-style threads or curious about alternative solutions to this issue, it might be worth checking out.