I have read that this change was not made by Canonical, but upstream
by Xorg. It applies to all distros that use the latest xorg, not just
Canonical ones.
The solution is to re-enable it with dontzap --disable or get used to
the new keys, Right-Alt+SysRq+K (for kill). It is probably better to
get used to the new keys as you will always have to re-enable the old
ones if Xorg changes and it is universal across all distros. Goodbye
Ctrl+Alt+Backspace!