I think Mattermost lost a lot of instance admins' trust when they recently decided to update the server to limit access to old messages without good reason. On self-hosted instances!
That's a shame, I interviewed there once, decided not to take it but it was one of the few places I could have seen myself working at, they seemed like decent folks trying to build something worthwhile.
If you work with lots of other entities who want full control over their own comms (e.g. other governments, other departments, other EU entities like European Parliament and Council, the UN, NATO, etc) then decentralisation or federation is a big deal.
In the public sector it's basically a requirement: it's bananas if your country's critical infrastructure ends up dependent on some a product effectively controlled by another country (e.g. Teams) - and you obviously want to be able to communicate with other govt entities rather than being stuck in an island.
Then it's a natural extension to the private sector - although for now, it feels more folks are on the "nobody got sacked for using Teams" train.
Well there's always Matterbridge. If you don't have complicated workflows to replicate (and even then) you can just replicate to XMPP, Nextcloud or whatever.
