Problem
The current proposal on T215672 for enabling opt-in and out of Growth emails requires users to be logged in to change their preferences, which may not comply with CAN-SPAM Act guidelines.
Proposed solution
Create a Special page with a token that enables the user to unsubscribe or change email preferences via clicking on an "unsubscribe" link in the email without needing to be logged in.
Example unsubscribe page from Wikimedia Fundraising emails: