It does. https://zulip.com/help/general-chat-channels
But even without that, in a normal threaded channel, you can see all messages in all topics chronologically. IIRC that's the default view when you click a channel in the sidebar.
Here's an example: https://chat.zulip.org/#narrow/channel/138-user-questions the messages are grouped into runs of the same topic, but it's the whole channel.
https://chat.zulip.org/#narrow/channel/101-design/topic/UI.2...
See also my and other users feedback about their UI in: Zulips values (24 comments) https://news.ycombinator.com/item?id=46953815
For example, if you had it open on your laptop with a window open, suspend it, and open it up on a plane, you can read the last few weeks of message history, compose replies that will send when you regain network, etc. I do this regularly on flights.
We always have ideas for how to improve this further, and the mobile app doesn't do as extensive caching as the web app does, but it's not an issue of technical feasibility. The protocol was designed for mixed online/offline use from the beginning.