Most of those examples would break today if run on Windows, though (AttributeError). So they'd just continue to break, probably with a different error (I'm not clear what happens if we specify SOCK_DGRAM with this change).

Having an undocumented field doesn't really help much - what we want is a documented field with a different name, perhaps WIN_AF_UNIX? (I think we need to keep the "AF_" prefix for real values and not mess up the namespace ourselves.) That way we can document that it matches AF_UNIX when defined on Windows, but will not trigger existing code that checks for the presence of AF_UNIX. Then we'll have to update the standard library to use either/both values where supported.