chrome notifications unsubscribe event
Your looking for the pushsubscriptionchange event
self.addEventListener('pushsubscriptionchange', function() { // remove the entry from DB});
Unsubscribe is browser setting that can be done by settings tab and offline mode so you cannot get that information
But Once the user has unsubscribed (i.e. deleted the service worker registration), then on sending the next message you should get a failure message from the FCM API.(if you are using firebase)
So think in this way .... :)