It's a good one (Signal as well, though). My favorite design decision was to tie it into the email ecosystem, so if anyone tries to block it, they will have to block email, which their business buddies won't be happy about.
Not federated, but definitely anonymous. All Signal messages are E2EE, and Signal can't even access your messages. They literally have a page where they list every time they've been asked by the government to give info, and they can't.
https://signal.org/bigbrother/