You're right, it appears Google's new policy is to make the List-Unsubscribe header mandatory, and require certain semantics[0] for the URL it contains, so that

You're right, it appears Google's new policy is to make the List-Unsubscribe header mandatory, and require certain semantics[0] for the URL it contains, so that they can issue unsubscribe requests on behalf of users in an automated fashion: https://support.google.com/mail/answer/81126#subscriptions https://support.google.com/mail/answer/81126#subscriptions (This wasn't immediately clear to me from the original link or the Google blog post.) [0]: https://datatracker.ietf.org/doc/html/rfc8058 https://datatracker.ietf.org/doc/html/rfc8058