On Zulip you can move messages to another topic[0]. Streams are the concept of "channels" on Zulip, and Topics are your threads. On discord, if you use the reply feature enough times, (iirc 3-4 times) it'll prompt you to make a thread and have those messages included iirc.
Thanks. Didn't see that in zulip, but haven't used it 'for real' beyond a few demos a couple years back. I don't remember getting prompted in discord like that; is that a setting someone needs to enable?
0: https://zulip.com/help/move-content-to-another-topic