So for targeting X11/Wayland or Windows, you can bet 100% on C++, only Apple platforms do not offer native C++.
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
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.
On modern Windows you have C++, and also layout DSL, specifically XAML. And that C++ is not particularly idiomatic, it’s IUnknown-derived interfaces.
If you mean windows desktop, WinAPI uses quite a few DSLs. Dialog and menu resources are the most obvious, but there’s more: WMF/EMF vector graphics format, ribbon markup XML for ribbon control, theme files…
Modern Windows desktop UI, UWP aka WinUI, uses C++17 with a framework called C++/WinRT, that hides those IInspectable COM interfaces.
Yes there used to exit C++/CX and Windows Runtime Library, both now considered legacy and replaced by C++/WinRT.
Microsoft is the only consumer OS vendor that still supports C++ for doing full stack GUI apps, everyone else moved into a mix of C++ plus managed language or layout DSL.