Currently, language changes are applied with the new Qbs version.
It is much better to check the Qbs version required for the project. That way, users can update their project as well as bumping the version in the product; otherwise, Qbs updates do not break things.
We can also check the default values of properties based on the project version.
Unfortunately, this cannot help us removing legacy code/properties
This is also in-line with how CMake version policies work.