This is straight up extortion. Either you start paying for a "free forever" service, or you lose access to every account you registered using Google OAuth with your custom domain email address.
You can migrate your domain and email, but you CAN'T MIGRATE your Google OAuth, and without that you lose access to all your accounts ON OTHER THIRD PARTY SITES.
You can migrate your domain and email, but you CAN'T MIGRATE your Google OAuth, and without that you lose access to all your accounts ON OTHER THIRD PARTY SITES.