While some buildings make it more obvious by workers leaving and walking around outside when they can’t work (usually due to resource not being available), others like for example the Foragers just sit in the building, giving you no indication that they used up all the resources in their assigned area.
Thus an icon that shows that a production building is missing resources to continue working (unless they are currently being delivered by a stocking worker) would be useful.
This already exists for other buildings, like guard towers after all.