THAT IS IT! I was wondering why the name always felt funky. Although the name they have now is technically correct if you ignore standard noun to adjective conventions.
E.g. having windows 1st sort of inflates the 'primacy' of windows with respect to linux; it is linux that is being made compatible with windows rather than the other way around.
Best things that come to mind are things from the windows UI - like the network privacy zones, having the 'basic' and 'advanced' control panel, etc.. Some things say 'windows is updating your computer' rather than 'your computer is updating windows' or some such... To me they have this sort of psychological undertone of 'always remember how microsoft is helping you make this confusing thing much easier.. you couldn't do it without us'
a. Windows' Subsystem for Linux
b. Window's Subsystem for Linux
For the possessive form of Windows I would look at the Microsoft Manual of Style, which for me is the fourth edition.
Page 184: Possessive Nouns
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Do not use the possessive form of Microsoft. Do not use the possessive form of other company names unless you have no other choice. And do not use the possessive form of a product, service, or feature name. You can use these names as adjectives, or you can use an of construction instead.
Microsoft style:
the Windows interface
Microsoft products, services, and technologies
Word templates
That is correct (or, at least, nearly universally agreed to) for possessives of plural nouns ending in “s”, style guides are mixed when it comes to other nouns ending in “s”, though the most common rule seems to be to use “’s”; while “windows” is plural, “Windows” as the name of the operating system is a proper noun that is not treated as plural.