Globally unsubscribe a contact from all email marketing. Identify the
contact with email or phone_number (not both).
If the contact is already unsubscribed, this endpoint returns successfully without making changes.
Required scope: contacts_write
OAuth 2.0 client credentials flow. Exchange your client ID and client
secret for an access token at the token URL, then include it as
Authorization: Bearer <token>. See the Authentication page for details.
Identify the contact with email or phone_number (not both).
Contact unsubscribed successfully.