The encryption key is stored in the keychain. Most keychain items can be easily accessed with various phone breaker software.
Any device made by a large company cannot be guaranteed to be secure,unless you make your own hardware and software and communicate via encrypted protocols. Only then you can guarantee that your systems are safe. Even new encryption is intentionally designed not to be 100% secure.
The access to Whatsapp keys does enable access even to deleted messages. There are two ways for a user to delete WhatsApp messages. The messages can be deleted one by one or cleared all at once by using the clear/delete chats button. Messages deleted in either of the ways mentioned above can be recovered because deleted messages are stored in msgstore.db-wal, which is the cache file for message storage. You just have to access the cache before accessing the main DB to prevent any possible over-writes of it.
Therefore, Smart Phone Forensic Systems and Phone breakers support fully the recovery of keys and any data that was encrypted with them, including deleted data.