i think because you dont see the whole picture.
script is build on https://wiki.multitheftauto.com/wiki/GetVehicleType
meaning the stuntplane would require a seperate cancel/call function in the script.
your vehicle is deleted to make space for other vehicles,
After you go inactive for longer than 1 month your vehicles will automatically be removed due inactivity.
You are free to re-request your vehicle on the " Donation reward change " section.