France’s government uses Matrix. Presumably a nice perk to be able to talk to them via federation

I'm not sure if it ended up being used this way, but if I recall correctly, when that was being initially implemented, federation was actually a core feature: different agencies / municipalities / etc could have their own servers and control their own data and accounts, but inter-agency conversations and rooms would be well-supported, along with each agency retaining a copy of the rooms on their own servers.

Germany (government and armed forces) and NATO also use it.