[maxwellg]

This is conceptually extremely similar to the Web Push API: https://web.dev/articles/push-notifications-web-push-protoco...

You'd need something at the browser/UA level to unsubscribe or to make the subscription exist for only a single message. Bad content publishers have taught us to never allow Web Push notifications since they always get inundated with marketing and other nonsense - being able to bake protections against that into the spec could be interesting.