I don't think the ReactOS people are trying to do Windows versions like Russian nesting dolls. They try to aim to fully support the older version, but while the

I don't think the ReactOS people are trying to do Windows versions like Russian nesting dolls. They try to aim to fully support the older version, but while they're trying to fill all the nooks and crannies of them, quite often you'll find yourself adding some new API from a more recent version, if that's all you need to support a specific kind of application. I'd naively assume that finishing the undocumented API calls of the last century is a much more time-consuming task than adding what little came after 2003...