In a little more than 24 hours I will stream GraphQL Subscriptions through the looking glass - A reflection on GraphQL Subscriptions and four server implementations on the JVM on YouTube,
https://www.youtube.com/watch?v=QRBgKWSqJog▾
(despite the description in Dutch it will be in English).
Seeing this 5 months later. The Kotlin one is relatively new. And the Micronaut one I added myself. The Clojure one is a bit older, but you probably didn't consider Clojure.