https://doc.qt.io/qt-5/topics-app-development.html still links to "Qt Quick Compiler". Anyhow, the documentation of the tool says

Since Qt version 5.11, the functionality of the Qt Quick Compiler has been integrated into the Qt Quick module itself. This separate add-on remains available in the earlier long term support releases of Qt.

Since Qt 5.11 or older versions are not supported anymore we should arguably remove the link.