As in HTTP defined you should return 410 if a resource is no longer available.

410 Gone
Indicates that the resource requested is no longer available and will not be available again. This should be used when a resource has been intentionally removed and the resource should be purged. Upon
receiving a 410 status code, the client should not request the resource again in the future. Clients such as search engines should remove the resource from their indices.[44] Most use cases do not require
clients and search engines to purge the resource, and a "404 Not Found" may be used instead.