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OpenAPI has a handful of open source API explorers. The ones I’m familiar with are Swagger UI, Redoc, and RapiDoc.

OpenAPI 3.0 has this concept of remote references, which can be URLs to other OpenAPI specs hosted anywhere. https://swagger.io/docs/specification/using-ref/

It may not be an exact analog, but "swagger UI" for openapi is the best I've seen like GraphiQL. Example https://petstore.swagger.io/ . Not sure of other alternatives.

No analog for "subgraph federation systems", unless a load balancer will suffice.

What are the use-cases for graph introspection in any but a tiny fraction of cases?