Assets are never completely deleted, only deactivated. If you are already using an asset in an AdSlot, it will remain active in that AdSlot. However, it is no longer available to fill new AdSlots.
Important: In the asset view in the AdSlot, you cannot currently see whether the selected assets have been deleted in the meantime or not! Therefore, before or during the asset upload, make sure you have a clear taxonomy so that you can easily distinguish between the assets later on.