to some degree graphql provides basically rpcs for creating subscriptions, but doesn't really care about the mechanism those subscriptions are provided over

I think AWS gateway supports GraphQL subscription based on AWS lambda. Never used it myself. I did the Websocket implementation for Micronaut. Would be possible to do something using RSocket or gRPC. Problem is, like with websockets, lack of a standard, so you need a custom server and a custom client.

There as some community efforts towards speccing those, but it goes slow..