My chair at TUM uses Mattermost for most internal communication. I'm aware of a couple different academic institutions that do that.

EDIT: Perhaps of important note, we're talking Computer Science systems people. Like, kind of an environment where you're professionally obligated to have strong opinions about Linux distributions.

In all cases it's self-hosted. I don't know anyone who bough hosting. If I get my hands on our sysadmin I could share more.

As for "switched" - I was at one company only that had an official/unofficial Slack and we were given stern talkings to about never ever sharing any company information through it. It was specifically for watercooler banter. Basically "treat it as a personal non-work environment". Specifically because our security officer was aware that Slack is not end-to-end encrypted and that's just an immediate dealbreaker in any sane company that handles sensitive data. God I miss that CSO, I didn't know how good I had it.

Microsoft has Yammer because they always need to choose the worst possible tool in existence.