I reckon somewhere out there, is some obscure program used to control some piece of scientific or industrial equipment, and it talks to that equipment over COM1, and it is just opening "COM1" as opposed to "COM1:" or "\\.\COM1", and it still runs on Windows 10. Rare scenario, few people are going to be impacted by it, but probably somewhere out there is someone who will be. And making it a registry key or EXE manifest option helps to not break that rare software. And I wouldn't assume that systems connected to laboratory or industrial equipment don't get Windows updates; some don't, some do.