JULY 30, 2021 UPDATE: This version is now outdated! Please go here for the latest.
Unsubscribe Link 2.2 (OUTDATED)
Allow Recipients to Unsubscribe From All Email Sent via Salesforce
When the recipient clicks to unsubscribe, a flow will look for all contacts and leads who have this email in the Email field and mark them “Email Opt Out.” The email address owner will receive one confirmation email immediately. A custom object tracks when someone unsubscribes to allow you to report on it.

See How It Works
UPDATE: 1/28/21 I realized how to send the confirmation email from someone other than “Site Guest User.” More in this post.
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