Slack channels are largely defined by _membership_, not necessarily by _topic_. At least in my organization (we use Slack), there is generally a channel (or channels) for each team and larger channels for the whole company.

As these channels get bigger, they inevitably split into sub channels (#myteam becomes #myteam-status, #myteam-oncall, #myteam-eng, ...). But this is clunky and can lead to a lot of confusion as to what chats should go where. Furthermore information retrieval can be cumbersome as I’ve found myself flipping through 5 chats and searching them all for a conversation.

I don’t have any experience with zulip, but it’s strategy of partitioning conversations largely aligns with how I think when I look for things on Slack: First I recall who I was talking to (channel), then I drill down and look for the thread I’m thinking of.