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.
That was not very obvious from their landing page!
Well in that case, carry on!