There's also the more recent Alta and Solitude from Libre. They seem to be supported in recent CoreElecs nightlies.But yeah, information on those is also a bit thin.
The funny thing is, that the Bundeswehr actually has a communications platform based on Matrix: bwMessenger goes live for Bundeswehr, element.ioWhy they're not using it? Who knows...
There's also the more recent Alta and Solitude from Libre. They seem to be supported in recent CoreElecs nightlies.
But yeah, information on those is also a bit thin.