Would adding federation to Signal help with users behind country-wide blocks? Seems like a distributed service would be harder to censor than a centralized one.
If the censors somehow didn't hit every single worthwhile federated endpoint, users would still be left wondering why they couldn't communicate with most of their friends. Moving between federated hosts would also necessitate an entirely new identifier, so users would need to rebuild their social graph again.
In addition to being ineffective against censorship, there are several other properties and trade-offs that make federation a difficult proposition for an application like Signal: https://signal.org/blog/the-ecosystem-is-moving/
That's not how federation works, at least in XMPP. You only need to connect to one server that's out of censorships' reach to be able to communicate with everyone.
While it's true that my friends on other servers might be able to send messages that will arrive on my chosen server, that distinction isn't very meaningful because I am unable to connect and retrieve those messages.
I wouldn't be communicating with my friends until I switched to a new server and rebuilt my social graph.
Also, while it's not specified in XMPP (yet), it's easy to imagine a federated service that lets you connect to any server in the network that then behaves as a proxy to whatever server you have your account on.