The motivation for this feature request is explained in this question.
Edit to add more detail : When I click my user name at the top of Stack Overflow, the info tab is the default. Then select prefs tab lower down. Bottom right is a link to tag subscriptions. Over on the bottom left is a heading "Miscellaneous" with one option: "Allow email notifications when I subscribe to questions or tags". Could a new option be added there (ticked by default to match current behaviour): "Include a one-click no-login-required unsubscribe link on notification emails".