Rooms and areas can be deleted in model views like floor plans and ceiling plans. Revit warns us that they have been deleted from the model but they are still listed in schedules. To really delete them we have to open a schedule view and delete them there. That is intended to make it easier to place them elsewhere in the model without having to recreate them from scratch if their current location needs to be changed.
We can also create rooms and areas in a schedule view before there are any walls or boundaries available to define where they should go. Revit just reports "not placed" until we put them somewhere. We can do the same thing with sheets, they are called placeholder sheets.
The contrast or inconsistency compared with rooms and areas is that you can delete the sheet from the Project Browser and it is removed from the sheet list (schedule). It wouldn't be too unexpected to think that deleting the sheet should leave it in the database as a placeholder, kind of like rooms and areas?