I am still receiving emails about failed deploys for an application that I have stopped working on.
Our notification system relies on GitHub for some actions and occasionally there are synchronization issues that mean a user is still subscribed to alerts.
To reset the notifications yourself, you can visit https://dashboard.heroku.com/account/applications, and click the button to disconnect from GitHub ("Revoke access..."). Please note this will unlink ALL apps from GitHub, which might include some that you still have access to. You would need to reconnect and re-link the apps which you wish to keep linked to GitHub.