The KDE desktop shell is QML + C++. The vast majority of KDE applications are QWidgets + C++ without QML.
I like QML but it's not that great for many / most desktop applications. It works better for creating fully custom touch UIs from scratch. The plan for Qt 6 mentions a styling API which is one of the most important missing pieces, and discussion in the comments maybe indicates more ready to use controls.
Which UNIX OS SDK uses a C++ GUI full stack?
One could argue for Kubuntu or SuSE, maybe, except KDE isn't tied to a specific OS and then again it uses Qt alongside QML since Plasma, so no pure C++ stack here.
And since you missed to fully read my comment, here it is again:
> Microsoft is the only consumer OS vendor that still supports C++ for doing full stack GUI apps