In the past Google and Facebook supported xmpp(jabber) protocol. That was real step in right direction. You could use your client and OTR plugin to encrypt all communications. It all ended in 2015.
(I think a lot of useful Signal properties are much harder to do with federation, but that’s a subtle enough problem that it warrants a long form post, not a HN comment. I agree that ceteris paribus federation is better than not—but c.p. is doing a lot of work there :))
Is there some reason to believe it didn't happen (mostly) for the reasons stated by the respective parties at the time? 'federated' is often brought up as an unalloyed good so casually but as you point out yourself, there's a huge overhead, conceptually, operationally, etc.
