post https://api.onesignal.com/apps//notifications//unsubscribe?token=
Unsubscribe an email with a token when using your own custom email unsubscribe landing page.
Unsubscribe an existing subscription identified by their unsubscribe token. The unsubscribe is counted against the notification that belongs to the notification_id that is provided in the request URL, which will increment the total unsubscribes and record an unsubscribe event for that notification.
The token can be generated when crafting your email's HTML body by using liquid syntax, and should be used when bringing your own custom unsubscribe URL.
Follow this guide to learn more about building your own custom unsubscribe page.