It's enabled by default in qt6

src/kdf.cpp

@@ -48,7 +48,9 @@ void KDFTopLevel::closeEvent(QCloseEvent *event)
 int main(int argc, char **argv)
 {
     //Fixes blurry icons with fractional scaling
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
     QGuiApplication::setAttribute(Qt::AA_UseHighDpiPixmaps);
+#endif
     QApplication app(argc, argv);
 
     KLocalizedString::setApplicationDomain("kdf");