Zulip is my favorite Slack alternative, but this Discourse forum might be even better for the following reasons:
- Discourse allows notification control for each thread/topic. Zulip has this feature on the wishlist Subscribing to topics within a stream for push notifications · Issue #6027 · zulip/zulip · GitHub.
- Discourse does not require sign-in to view content.
Some more discussion comparing Slack, Discord, Zulip, and Discourse is in: Discussion platform with full message history