The previous release, in 6.3, was based on a prototype code, in 6.4 we will have a reworked implementation. We still will treat it as Technology preview until 6.5 because it needs time to be tested in field. In 6.4 and in 6.5 the type compiler will still be an "opt-in" feature that needs to be explicitly enabled in the application build, despite other additions to loading of QML on the C++ side. See https://doc-snapshots.qt.io/qt6-dev/qtqml-tool-qmltc.html