I learned about this http response code too late. About 4 years ago I was working at a startup and I was the "lead engineer" (aka only engineer) on a project where I had to design and implement an entire REST API. I really wish I would have put this in somewhere, since we weren't doing code review (because it was literally only me).
Bit disappointed that this is not built into the c# http status codes. Was building a mock service and wanted to return something that would never occur in production for things I didn't have definitions for. This seemed like a perfect response but it's not part of the statues enum.