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Which Windows version are they emulating now?

It's not possible to say that precisely; the closest thing I could find is this: https://www.reactos.org/wiki/Missing_ReactOS_Functionality .

On the outdated page https://www.reactos.org/wiki/Version_Status you can see that Windows 2003 (the server version of XP) is a bit of a target.

Windows 2003 sp2