But the standard Free Software development/distribution model does lack in some areas. And so Signal got a bunch of community leeway for going against the grain, in the hopes that a fresh approach would somehow bear fruit.
We're now apparently seeing some of the fruit from that approach.
For notifications the alternatives are noticably worse (higher battery usage because you can't coordinate request timings with other apps, an annoying permanent notification), and the leakage is minimal. If you protect your encrypted packets from Google the NSA will see them anyway.
Your custom implementation will be quite complicated, and if you only enable it for a small subset of your users it'll be a pain to debug.
I agree about the sorry state of non-Google notifications on Android. I wish someone would make a common notification framework for the Free world that would be installed alongside system-level F-Droid. Although F-Droid Conversations and Element notifications do work fine for me, regardless of purportedly less battery life, I can understand not everyone wants to make the same choice.
However, I'm referencing more than the notifications issue. I recall an early thread from Signal where they touted the benefits of fully opting into the Google ecosystem - the gist was that Google has expended all of this effort on security and they wanted to take advantage of it to bring security to the masses. And that simply doesn't line up with my own threat model, in which Google is one of the most significant attackers.
What exactly do they rely on google for? They use them for their push notifications and they use some google servers on the back end.
They do offer the app on the app store as 99% of android users get their apps that way, but signal also offers app downloads from the signal website if the user doesn't want to use play store.