Comment 7 for bug 987026

It's actually ubuntu-release-upgrader that handles the distribution upgrade logic now and it does have some logic for dealing with specific packages. This can be seen in DistUpgrade.cfg:

# information about the individual meta-pkgs
[ubuntu-desktop]
KeyDependencies=lightdm, unity, ubuntu-artwork, ubuntu-sounds
# those pkgs will be marked remove right after the distUpgrade in the cache
PostUpgradeRemove=xscreensaver, gnome-cups-manager, powermanagement-interface, deskbar-applet, nautilus-cd-burner
ForcedObsoletes=desktop-effects, cups-pdf, gnome-app-install, policykit-gnome, gnome-mount

I'm not certain why cups-pdf is listed there but will dig into it.