> I think once Zulip gets a "guest mode", it will be a huge boon in its adoption for OSS communities

Agreed!

To clarify the terminology, "guests" in Zulip (and other team chat products, e.g. Slack) generally refers to users with an account but with limited access. https://zulip.com/help/roles-and-permissions has more detail on that.

We've been tentatively planning to use the term "web-public streams" for streams that an open community has configured to be accessible without creating a Zulip account. The project is quite far along; the latest PR is here:

https://github.com/zulip/zulip/pull/16728

(The PR looks stale, but that's only because we paused integrating it to focus on finishing 4.0; I would be sad if we don't have it at least in beta this summer.)