These look like a weird mix of Slack threads (temporary (though not enforced in Slack), written in sidebar) and Zulip threads/topics (named, available in the channel list), though... with downsides compared to both (for example, you can't forward messages to original channel like you could on Slack). Feels a bit awkward, but time will tell.

My main concern right now is with discoverability and moderation - it seems a bit difficult for moderators to see recent activity across all threads, unless I missed something.

Other than that, I'm cautiously neutral.