Zulip product lead here -- I'm not aware of anything in this space that used to work and no longer does.

There's long been a way to make Zulip channels searchable with zulip-archive (https://github.com/zulip/zulip-archive), and that continues to be available.

Making the app-native web-public channels accessible to search engines is a big technical project, tracked as https://github.com/zulip/zulip/issues/21881.