Do you think that extending TPageControl would be the best way to implement both nboShowAddTabButton and nboShowCloseButtons behaviors on Microsoft Windows?
I am not familiar with the Windows pagecontrol widget (and use Windows only when I have to, otherwise I avoid it), so I don't know if the Windows widget is capable of being "extended" to offer this functionality. If it is not so extensible, a Windows solution with closeable tabs would have to be custom-drawn, which goes against the LCL philosophy.
Usually if the underlying OS widget does not support some functionality, then a cross-platform implementation is simply unable to support that functionality on that particular platform. Hence the Restriction Browser etc.