push unsubscribe event or how to track unsubscription

I was looking at https://github.com/firebase/quickstart-js/blob/master/messaging/index.html and unless I missed something, there is no way to tell the backend if a previously subscribed user unsubscribes in future.

Is there an unsubscribe event that can be listened to on client and update server so such tokens can be deleted?

Start by reviewing messaging/index.html and the client token registration flow described in the issue. Determine whether Firebase provides an unsubscribe signal or API, then clarify the expected client-to-backend behavior and how completion would be tested; the issue currently defines no implementation scope or done criteria.

Written by the indexing model from the issue text.