The goal of the URL Standard is to reflect where all implementations will converge. It should not describe today’s implementations as that will not lead to convergence. It should not describe yesterday’s implementations as that will also not lead to convergence. And it should not describe an unreachable ideal, e.g., by requiring something that is known to be incompatible with web content.
This is something all documents published by the WHATWG have in common, but I was asked to clarify this for the URL Standard in particular. Happy to help!