Many of the server scripting packages/tools out there have the option to have a schema validation before the response is sent out to the clients.
fastify package for a
Node.js has a pretty upfront one.
Only two use cases that I can think of is restricting excess data and security. I feel for the first one, we are in a pretty good bandwidth era to not think about optimizing some 0.2 KiB of data. As for security, a developer who is going to expose the private keys in a restricted environment will definitely do that with a schema-less environment.
I know there cannot be a theoretically answer to this question, but I am looking at a practical answer. Personally, I don't like having it as I think I would use the time for something else productive. At least at server side code. Think it should be covered in unit testing.