Essentially, that is hardcoded. The package list (ie. apt cache) does not change after release. It cannot, because of mirror expectations and of course because

Essentially, that is hardcoded. The package list (ie. apt cache) does not change after release. It cannot, because of mirror expectations and of course because it ends up on the images. Updates go through a separate updates pocket. Essentially the "Supported" field cannot be updated for all packages without issuing an update for all packages, which would require all users to download all packages again to get the new versions of each of them.