The fact that Reddit can still tell you that the user deleted their account is proof that not all of that user’s data in their systems are deleted. It may just be a flag in an account that marks them as “deleted”, and so whenever data about that account is being retrieved, their API server will look at that flag, and tell the recipient that the account is “deleted”. People in the software industry calls this “soft deletion”.
The fact that Reddit can still tell you that the user deleted their account is proof that not all of that user’s data in their systems are deleted. It may just be a flag in an account that marks them as “deleted”, and so whenever data about that account is being retrieved, their API server will look at that flag, and tell the recipient that the account is “deleted”. People in the software industry calls this “soft deletion”.