Plenty of apps on Android are great replacements for centralised services we've gotten used to, and can be installed from another source like fdroid, like clients for Telegram, Matrix, Lemmy, Mastodon, Mattermost etc. As they weren't installed via Google Play, they can't use Google's notification service and instead use local alternatives.

They do not allow that, but yeah, it’s just their OS which only allows access to the relevant system interface for their own app. Apple doesn’t let you send SMS with third-party apps either for example.

Though admittedly, Google is putting proprietary extensions on top of it in their client, and they are apparently running a lot of carriers’ RCS endpoints, and using their servers when the carrier doesn’t support it at all. Which is fair, but imo does not make RCS itself inherently proprietary.

(However this is also to some extent warranted, since carriers were and still are dragging their feet a lot implementing it despite RCS being a required part of 5G carrier services IIRC^1^. This seems to me like another IPv6 situation.)

This claims to work on a rooted Android phone (or one where you have control over the system image), and the underlying library is platform-independent so you could use it to implement RCS for a Linux or other phone: https://github.com/Hirohumi/RustyRcs. I haven’t tested it though since I also don’t Android (anymore).

^1^ Though maybe that was just for 5G standalone, which no carrier is doing yet anyway.