Slack has basically one main hierarchy level (messages are grouped into channels) while Zulip has two, streams and topics. So you can create a stream for each project (say) and create a different topic for any given point that needs discussion about that project.

Kind of like if each slack thread discussion had a title and was discoverable from the left sidebar and didn’t get in the way of the other threads.

> didn’t get in the way of the other threads

But also, critically, if you want to, you can drop back to the "show me everything sequentially" view. Threads hide discussions away - which is good when you want to focus on something else, but bad when you can't remember where a discussion was.