It's OSS and self-hostable. And it's got a great UI and the most joyous technology I've ever had the pleasure of using. https://zulip.com/self-hosting/
That was not very obvious from their landing page!
Well in that case, carry on!
It says in bold letters:
"Your data is yours!
For ultimate control and compliance, self-host Zulip’s 100% open-source software"
I guess I've been on the internet too long, my brain automatically blacks certain language out, like a biological spam filter.
...You could go to the Slack website right now and see? We're on the internet. It's all on the internet. We can literally just check.
Doesn't seem to mention anything about being open source, anything privacy-related, data, or hosting.
> When you self-host Zulip, you get the same software as our Zulip Cloud customers.
> Unlike the competition, you don't pay for SAML authentication, LDAP sync, or advanced roles and permissions. There is no “open core” catch — just freely available world-class software.
The optional pricing plans for self-hosted mention that you are buying email and chat support for SAML and other features, but I don't see where they're charging for access to SAML on self-hosted Zulip.