apache kafka – Alternatives to the REST API for external services

I'm looking for alternatives to the REST API and I have trouble finding information about whether people do it and how they do it to expose services to third parties. Suppose there is a service that requires large complex / nested loads to be sent from client to client for processing.

While there is no reason why you can not expose your service to an external client as if it were an internal microservice of yours, it seems that I can not find examples of this.
For example, if your service reads from a Kafka cluster to do the processing and your B2B client can write to the cluster. Or, expose a gRPC server to your client. Both get the benefit of the compact binary message format for those large payloads versus JSON / REST. And instead of the typical Open API documentation, does it provide an SDK? I have seen putting the grpc server behind a REST API, but not completely from third parties.

Any thought very appreciated. Is this even feasible? Are people doing this?