I don't suppose there's such a thing as an open push notification service that could work on iOS, Android, and security-focused mobile OSes such as GrapheneOS,

I don't suppose there's such a thing as an open push notification service that could work on iOS, Android, and security-focused mobile OSes such as GrapheneOS, is there? Lacking such an ability are websockets or polling the only options for push-like notifications on an OS like Graphene?