https://brutecat.com/articles/leaking-youtube-emails
the interesting (for me) part of this attack is not the leak of the google ID (gaia ID), or the usage of a legacy system.
the interesting (for me) part is the trick they use to prevent the notification email (which would alert the victim) from being sent:
i guess the action is executed first, the notification email is sent afterwards and when an email sending error occurs, the action is not rolled back.