Read We can include Signal in F-Droid and those links posted there.
A program is free software if the program’s users have the four essential freedoms:
From here: What is Free Software? - GNU Project - Free Software Foundation
- The freedom to run the program as you wish, for any purpose (freedom 0).
I can’t compile AND connect to their servers, right?
- The freedom to study how the program works, and change it so it does your computing as you wish (freedom 1). Access to the source code is a precondition for this.
We have this? Maybe, but since there’s no federation I can’t swear the Signal server is not doing anything funny.
- The freedom to redistribute copies so you can help others (freedom 2).
Not possible to since Google Services are not FOSS (well this is linked with the one below in the case of F-Droid)
- The freedom to distribute copies of your modified versions to others (freedom 3). By doing this you can give the whole community a chance to benefit from your changes. Access to the source code is a precondition for this.
And the reason for these endless threads, F-Droid can’t host a modified (truly FOSS) version.
So add the good points, subtract the bad ones, is the result 4 (four) freedoms or less? If it’s less…
/PS: I’m no lawyer so yeah 
Really, all you should read is: Signal >> Blog >> Reflections: The ecosystem is moving
The Conversations dev rebuttal: An Objection to The ecosystem is moving (I’ve recommended Conversations or forks above because the ecosystem moved in a good way)
And the comments on the 2019 Moxie rehash of the same text: https://blog.jabberhead.tk/2019/12/29/re-the-ecosystem-is-moving/
(fun fact Moxie took down the Youtube video of his own talk for some mystical reasons only to end up referencing if from Peertube a decentralized system later on as he needed to make some Twitter justifications of Signal’s development)
Also, did you see we host the Telegram client? That’s ironic…