Active use is really irrelevant if you only plan on using it inside a company, because you can implement a client and server within 30 mins to an hour, no external tools needed. The spec is clear and readable. It's excellent. We use it with a TypeScript codebase and just share the interfaces for the services in a monorepo. The spec is so simple it doesn't really need an update.
If you want a more advanced implementation with type inference for Typescript you can use this: https://github.com/shekohex/jsonrpc-ts
I'd still recommend implementing your own, my own implementation is based off the one above.
The only caveat compared to GRPC is you lose out on field validation due to it not being protobuf/and the obvious json decode overhead
Edit: if you want to know some current users of it, I believe the Ethereum protocol uses it heavily