The Google Cloud Platform Terms of Service (section 1.4(d), "Discontinuation of Services") defines the deprecation policy that applies to App Engine. The deprecation policy only applies to the services, features, or products listed therein.
After a service, feature, or product is officially deprecated, it continues to be available for at least the period of time defined in the Terms of Service. After this period of time, the service is scheduled for shutdown.
For information on the lifecycle of each runtime, see the Runtime support schedule.