A "sticky" window is one that stays in the same location on all desktops. This is one missing feature which makes it hard to write something like conky (or any kind of desktop widget) in pure QML. Rendering directly to the root window is obsolete: the modern way would be to have a transparent window which is stuck to the root window, assuming one has a compositing window manager. So I think we need to have another window flag alongside Qt::WindowStaysOnBottomHint etc.
But it requires some research whether this feature can be supported on any platform other than X11.
The _NET_WM_STATE_STICKY flag is not found anywhere in Qt source code, neither in qtbase nor in qtx11extras.