OpenAPI is good for RESTful services and at the moment, I'm hacking it to do it for asynchronous messaging system (specifically Kafka) by using POST to a /topic so that I can use redoc do create a website for the API.
I am trying to see if there's already established system of documenting for this. Especially since the GET /events which is used for event sourcing is getting larger and larger by the day.
It seems asyncAPI is basically what you are looking for: openapi but for topics instead of REST endpoints.
https://www.asyncapi.com/docs/getting-started/coming-from-openapi/