Both are serving the same purpose, for different apps. Even before we get to network connections and servers (the things that might make sense to be proprietary), without Google's ecosystem the local side has a bad UX and requires hacks.
That is just because they are not using something like https://bubu1.eu/openpush/ and instead implementing their own push services. This is not a problem with Android but with the apps.
"Telegram" / "Push service: tap to learn more" (tapping opens https://github.com/Telegram-FOSS-Team/Telegram-FOSS/blob/mas...)
"Signal" / "Background connection enabled"
Both are serving the same purpose, for different apps. Even before we get to network connections and servers (the things that might make sense to be proprietary), without Google's ecosystem the local side has a bad UX and requires hacks.