The Qt Quick Controls 2 module has been moved into qtdeclarative.git.
The motivation for the move is explained here:
https://bugreports.qt.io/browse/QTBUG-79454
There was then a contributor summit session:
https://wiki.qt.io/QtCS2021_-_Testing_upstream_changes_with_downstream_modules
The conclusion from this was that the best solution would be to move qtquickcontrols2 into qtdeclarative.
To inspect the code in the dev branch prior to its removal, follow these steps:
$ git checkout -b my-dev dev~1
Switched to a new branch 'my-dev'