QML extension plugin documentation needs an update

https://doc-snapshots.qt.io/qt5-5.15/qqmlengineextensionplugin.html

Talks about the old method of creating extension plugins; deriving from QQmlExtensionPlugin and overriding registerTypes(). The code snippets and example projects are ported to the new method, but not the documentation itself.

The same applies to this tutorial:
https://doc-snapshots.qt.io/qt5-5.15/qtqml-tutorials-extending-qml-example.html

Edit: Partially fixed in QTBUG-81615. Tutorial remains.