Unfortunately you are right, but the “Save Game” flag was definitely a good solution (but not fully implemented).
“My” solution is revealed after you want to save the item after you dropped it in the level in a new place.
Thanks to serialization, you will load ALL (the ones you need) its parameters by calling one universal function, without thinking about the fact that the classes of objects are different.
But this is only PART of the save system. The part that monitors whether the level object has been collected is not directly related to serialization.