> But there was no option in the MacOS Mail client to unsubscribe.

The functionality for mail clients to offer an "unsubscribe" button is dependent on there being a "List-Unsubscribe" header in the e-mail with a URL:

* https://datatracker.ietf.org/doc/html/rfc8058

* https://datatracker.ietf.org/doc/html/rfc2369#section-3.2

If the sender does not put one in then that's hardly the mail client's fault.