Well that’s the reason upstream Signal was not packaged on F-Droid, that it required Google Play Services to run. That’s why Signal was forked into LibreSignal (which didn’t change anything beyond removing this dependency) which could be distributed on F-Droid. [This ticket]https://github.com/LibreSignal/LibreSignal/issues/37) is where the discussion took place. m0xie from Signal team said:

I’m not OK with LibreSignal using our servers, and I’m not OK with LibreSignal using the name “Signal.” You’re free to use our source code for whatever you would like under the terms of the license, but you’re not entitled to use our name or the service that we run. (…) It is unlikely that we will ever federate with any servers outside of our control again, it makes changes really difficult. (…) I understand that federation and defined protocols that third parties can develop clients for are great and important ideas, but unfortunately they no longer have a place in the modern world

This discussion ultimately led to an article (and a CCC talk) called The ecosystem is moving, to which Conversations developer Daniel Gultsch replied. There was also a more XMPP-centric reply to the talk. Happy reading.