The GCM/FCM documentation is not clear on what would be the expected behavior, after April 11, 2019, for the apps that are already installed but not updated to use FCM SDKs.
Once GCM is defunct, will the old apps (having GCM SDK) continue to get new device tokens?
A similar question was asked here: Google GCM Deprecation - Old Mobile Clients with GCM Token
but did not receive a credible answer.