As a Ren'Py title, it should actually available for all three major platforms.
If you're using the itch app, though, it won't let you install it via it unless a developer marks the supported operating systems for each individual download on the itch site itself.
I'm sure eventually either the developers will figure out to do that by default whenever they upload files, or the "butler" system developed concurrently with the app will advance to a point that it can identify operating systems somehow, whichever comes first.