Hi Antonio,
Unfortunately you are correct. We don't backport significant features to earlier stable release branches because of the risk of introducing new bugs. We want to ensure that the stable release branches are safe to upgrade with minimal changes to the code or UI to ensure that people get the bug fixes we need without introducing potentially unwanted changes.
This is becoming more important as more people use the platform and build customisations on top of it. Just recently we had a situation were a seemingly benign improvement broke someone's site because of a local customisation.
If your client really needs the feature in 2.4 perhaps they might be willing to backport it once 2.5 is out (for their install only)?
Simon