There is a standard for this called list-unsubscribe (RFC 8058), but adding the header has no purpose. I have not been able to find an email client that uses the header: Thunderbird, FairEmail, Gmail, Hotmail, and I'm not sure if I asked someone to test it in Outlook but I think so. It is recommended to add the header for lowering your spam score, I don't know if it actually helps, but this one-click unsubscribe thing in your email client's UI seems to be a custom thing they add manually instead of just using the RFC...

References to show that this is not just me not understanding how to implement the rfc: https://stackoverflow.com/questions/4365850/list-unsubscribe... https://serverfault.com/questions/329032/how-to-add-the-list... I also checked emails that do show an unsubscribe button in the client's UI (such as a large pizza ad sender) and they do not have a list-unsubscribe header set at all.

If Google wants Gmail to be less spammy, they could start by not implementing double standards that give senders of big newsletters special treatment and a special hole in the spam filter.