I just started my de-googling journey recently, and so the mechanics of notifications were still unclear to me, and I found this video super helpful.
It explains how most mobile messaging apps (including privacy-focused ones like Signal) rely on Google and Apple’s centralized servers to deliver push notifications, which exposes vast amounts of user metadata.
Here’s the YT link, for people who prefer it: https://youtu.be/c3ennD3wKn0
That is correct.
However, this is a quasi-monopoly by google having quietly overwhelmed the space. Same thing for RCS messaging.
Neither push notifications nor RCS are proprietary, so there is a possibility to tear oneself from google here.
For instance, there are several free and paid push notifications services. Pushbullet is a popular paid one, not too expensive. I personally use https://ntfy.sh/, which can be self-hosted.
RCS is different because trusting the encryption keys makes RCS work, so there would have to be a critical mass of buy-in to use an alternative to google’s RCS implementation.