The Google Cloud Platform Terms of Service (section 1.4(d), "Discontinuation of Services") defines the deprecation policy that applies to Pub/Sub Lite. 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.