Gathering requirements doesn't have to take months. It can take hours or a few days. You can even do it in the middle of maintaining a product, e.g. as part of

Gathering requirements doesn't have to take months. It can take hours or a few days. You can even do it in the middle of maintaining a product, e.g. as part of steps towards addressing a feature request or a refactoring. It doesn't strike me as unreasonable that in the 50 years of building UNIX-style kernels, and the 30+ year development of Linux to have someone write down some functional requirements somewhere.