Were using Zulip. Before that we used Mattermost but we migrated to Zulip because of its (in our opinion) superior conversation threading model and never looked back.

Mattermost, Slack, etc. all work fine when you work on a project basis but when you work on a lot of short lived operational issues (for example "Issue with invoice XYZ" Slacks conversational threading model breaks down quite fast. At least it did for us.

iirc there is also a paid hosted version of Zulip but I don't know that for sure.

The hosted version is free for FLOSS projects and has a premium option: https://zulipchat.com/plans/.

We used mattermost in the past... maybe it's different now, but I remember the UI being frustratingly odd. Definitely didn't have the pleasant usability of slack.

Mattermost, works quite good. It is also pretty simple to set up and deploy.

[0]: https://github.com/mattermost/mattermost-server

EDIT: just read your post entireley.

>> 'My company does not want to use yet another free Slack alternative on GitHub'

So that one is not what you are looking for. But I keep the post in case anyone is interested in it...

Mattermost does have a paid (still self-hosted) version that includes support.

https://about.mattermost.com/pricing/

Nobody runs a SaaS for Mattermost?

Along the same lines as the question, does anyone consider keybase.io teams feature a viable alternative for small to medium sized teams?

We currently do use Keybase teams. We love the concept. Unfortunately, we do not consider Keybase production-ready, yet.

Especially the mobile apps are pretty buggy atm, which is one consideration why we’re looking for an alternative.

IRC?

"End-to-end-encryption" has flailed us.

microsoft teams

I'll second this. While it's not self-hosted, it is the only SAAS option that's actually usable in regulated industries. The main consideration is that the api is slightly different and some integrations aren't as nice (Looking at you PagerDuty)