QQuickWidget no longer works in Qt 6. This was ported to RHI at some point (as long as OpenGL was selected as the backend), but has since then regressed.
I tested with the following code on Linux and Windows (QSG_RHI_BACKEND=opengl), and the content is not displayed on either.
main.qml (added to resources):
On Linux, the window is completely black and on Windows, the window is completely black. On both platforms, the following error is displayed on the console: