Yep notifications require google play services installed. They do not require a google account or being signed in to one. It’s based on the device ID alone. I don’t have a google ID on my phone and push messages work just fine.
If you use on android apps from the playstore, most apps are built to relay their notifications through Google’s Firebase Cloud Messaging (FCM), so they don’t have to have a background notification listeners running 24/7. It also means less overhead from multiple notification listeners from every app having its own. So yes, if an app is built on top of google play services, it requires “google play services”
https://en.wikipedia.org/wiki/Firebase_Cloud_Messaging
If an app says it doesn’t rely on google play services, it uses an alternative notification listener or websockets, which might not be as effective, because Google is inbuilt and won’t kill its own apps, the scale of its infrastructure, and its habit of listening on people’s activity.
Add in Androids habit of killing background services, and you don’t always get your notification when the app isn’t in the foreground.
Even Signal from Playstore uses Google Play Services for notifications.
https://old.reddit.com/r/signal/comments/g217a6/what_can_google_glean_from_signal_using_fcmgcm/
There is also an alternative push protocol called UnifiedPush, but not many of the popular privacy focused messengers care to implement it.
