See also Microsoft's TypeSpec: https://typespec.io/ and https://learn.microsoft.com/en-us/azure/developer/typespec/o...

It's actively developed and used to spec Azure services, see https://azure.github.io/typespec-azure/ and all of the public Azure service specifications at https://github.com/Azure/azure-rest-api-specs (they're all OpenAPI for consistency, but newer development happens in TypeSpec, and the OpenAPI is an output).