I've been toying with the design of something similar here and there over the past couple of months. It never made it past the pen-and-paper stage though. Pleasantly surprised to find this today and will have to try it out for a work project.

My original motivation was the lack of OpenAPI definitions across projects, despite everyone agreeing their existence is a "best practice". As you suggest, developers just truly hate writing them. Unfortunately, even the "generate it from code" approach tends to be complicated and often feels duct-taped together.