Firebase Cloud Messaging (FCM) is the newer version of Google Cloud Messaging (GCM). So it is strongly recommended that new users (developers) to use FCM.
In relation to that, it is only possible to generate Server Keys by creating a Firebase Project:
Starting from September 2016, you can create new server keys only in the Firebase Console using the Cloud Messaging tab of the Settings panel. Existing projects that need to create a new server key can be imported in the Firebase console without affecting their existing configuration.
This is probably the note you've read. This doesn't really say that GCM is shutdown.