isn't the metadata from Matrix public? at least, if federation is on? seems useful to the OSINT community but I'd think public offices would have an issue with existing metadata about who is talking to who and when.
Yes it is, but I think you might be assuming that they are even putting their homeserver on the public internet.
But even if it is, then as long as federation is off, nobody else will see any metadata but them.
metadata from Matrix is never 'public' (i.e. visible to the public). It's visible to only the admins of the servers participating in a given conversation.