TL;DR toSet/toList are very convenient utils and they don't hinder new C++ standards integration.

I'm already sick of refactoring code like this (example of real code)

 #if QT_VERSION >= QT_VERSION_CHECK(5, 14, 0)
        auto tmp  = ms.sources + uris();
        auto tmp2 = QSet<QString>(tmp.begin(), tmp.end());
        _uris     = QStringList(tmp2.begin(), tmp2.end());
#else
        _uris = (ms.sources + uris()).toSet().toList();
#endif

toSet and toList were very neat. But now this code is hardly readable.

C++ standard lacks many cool things Qt has. Why do you at all want to make Qt that bad as plain C++?