A kitchen sink / dumping ground for any functionality that didn't fit elsewhere in Qt (at that point). Ends up stale over time, and discourages integrating the concepts in Qt in a cross platform way.
E.g. https://doc.qt.io/qt-5/qtwinextras-overview.html#type-conversion, QPixmap to HBITMAP and back. Similar functions for macOS. Nowadays lives with QImage, as QImage::toNSImage(), etc.
Should just use native APIs. Don't be afraid to mix and match. Qt can't (and shouldn't) do everything, and using Qt with other platform or 3rd party libraries should be possible and easy.
Similar to QtPlatformHeaders
Warning: This class is only available on X11. For querying per-screen information in a portable way, use QDesktopWidget.
Yepp, time to modernize