"Windows Subsystem" is a compound noun. It could serve as a noun phrase in a sentence like "Windows Subsystem is nice" or "I like Windows Subsystem", although without some article or other determiner (like "the" or "this") it doesn't sound very grammatical. Generally only mass nouns, like "dirt" or "food", or plural nouns, like "people" or "subsystems", can be noun phrases without an article or determiner. "the Windows Subsystem" or "a Windows Subsystem" (or "a Windows subsystem") would be (complete) noun phrases.