Furnaces should only accept smeltable items by hopper and manual input.
Smokers only accept food items in the "upper slot" and the blast furnace only accept the items it can smelt (like ores).
The advantage here is less confusion about what's smeltable, less clogging in furnaces arrays and the ability to filter non-smeltable items from the smeltable ones (which are then smelted).
For map-makers, this way of hiding items should not be removed, that's why it should stay that you can get items (also non-smeltable ones) into the smelt-slot via commands. Furthermore, the old furnaces with non-smeltable items in the smelting-slot just stay like that.
With this being a quite low effort to implement* without changing the core mechanics of the game, I can't see why it shouldn't be implemented.
*(This feature already exists for the fuel-slot, so it's "just" copy-paste plus adjusting)
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