I did have a backup of my music library, but it was a backup made by a backup program rather than simply a copy of my music folder. So once the tracks were removed, they disappeared from the backups too.
I now keep manual copies of certain folders as a separate backup to avoid any similar problems.
Still, the only content the iTunes program should ever delete are those governed by a subscription when the subscription expires. It should never delete anything else unless you tell it to, regardless of where you bought the content.
After hearing that Apple was abandoning QuickTime for Windows, I wondered how long it would take them to abandon iTunes for Windows as well, but then realized, they never will, because I'd be willing to bet that the vast majority of iTunes users are also Windows users.
Ah well, one can always hope.