BackSlot

One more equipment slot, on your back.

Using it

Drag something into the slot in your inventory, just above the shield slot. Or press G to swap it with whatever you're currently holding; the key is rebindable under Controls → Inventory, sitting next to vanilla's own offhand swap.

The slot takes any item and holds a full stack, exactly like the offhand does. There's no restriction on what goes in it. A sword, a shulker box, a stack of dirt, whatever you want on your back.

How it behaves

Nothing here is new to learn, because the back slot follows the same rules as the rest of your inventory:

• drops on death, and stays with you when keepInventory is on
• survives logging out, respawning, and coming back through the end portal
• destroyed by Curse of Vanishing, the same as any other equipment
• reachable in creative, in the survival inventory tab

Since other players see your back item, the mod has to be on the server too, not only on the clients.

No dependencies

Not even Fabric API. Drop the jar into "/mods" alongside Fabric Loader.

Limits

The keybind is labelled in English whatever your game language is set to. Translation can be added via the Github page.