Seeing as Zulip is growing ever closer to a chat/forum hybrid, I hope you’ll consider making Zulip instances browsable by guests, just like a Gitter or Discourse instance.
The master issue for that project is https://github.com/zulip/zulip/issues/13172. It's being worked on and is considered a high priority.
There's also an archive tool at https://github.com/zulip/zulip-archive.