I think it's reasonable to assume core developers are running a newer version of Windows. The constituency is now just 1, it's reasonable to have a higher baseline for developers, it's reasonable for us to not even officially document it for core development situations like this that are just dealing with legacy situations. I'd just add a code comment saying "Tested to work on Windows version xxx, known to not run on versions older than yyy due to not having tar bundled."
I added a force_local get parameter. Must be specified and = 1 to put --force-local in the tar command on Windows.