PendingUsernames: StorageDescriptor<[Key: Binary], [SS58String, number], true, never>

Usernames that an authority has granted, but that the account controller has not confirmed that they want it. Used primarily in cases where the AccountId cannot provide a signature because they are a pure proxy, multisig, etc. In order to confirm it, they should call [Call::accept_username].

First tuple item is the account and second is the acceptance deadline.