"Enhanced notifications" feature toggling :
unwat Short answer: it doesn't matter.
unwat Signal, for example, just makes a new notification with an FCM ID, a notification type, and the notification's priority. The data field is empty for messages. When a notification comes in, the app launches in the background, pulls down new messages, then the app displays the notifications itself. All Google can see is the user received a Signal notification.
Thank you very much for the in-depth explanations + links !
From these links, I found :
Developers already use HTTPS to communicate with Firebase Cloud Messaging (FCM). The channel between FCM server endpoint and the device is encrypted with SSL over TCP. However, messages are not encrypted end-to-end (E2E) between the developer server and the user device unless developers take special measures.
https://android-developers.googleblog.com/2018/06/project-capillary-end-to-end-encryption.html
So I guess Signal devs are smart enough to never display the message content within the notification ? (Idk I never used this app yet, please confirm it to me), but I've seen a lot over shoulders that apps like the facebook ones display the message content, be it as an app-at-rest notification
pop-up, or as an in-app data
pop-up ( https://firebase.google.com/docs/cloud-messaging/concept-options#encryption_for_data_messages ).
TL;DR So since the ever evolving "IA" part, regarding what answer suggest to what, runs on g2o servers I guess, I think it does matter to untoggle notification enhancements, on profiles running play services. Let me know if I got it wrong please
However now my trust issue is, what if someone uses a messaging app available on the playstore, but sideloaded from elsewhere than from the playstore, within a profile lacking the play services. Sure the user device won't receive the push notifications, but the devs servers will send the push notifications to the FCM servers anyway, to be decrypted in here anyway, before not encrypting it back + sending it to the end user as they don't run play services, right?
So for FCM servers not to see any part of the messages, one has to use apps that don't rely at all on FCM notifications = apps not present on the playstore ?