For the new fabric verion:

Right-click on the entitys/players, with the tether item, wich are supposed to be protected.

Right-click on entitys/players also removes the connections.

Shift + right-click to remove tether connections.

Connection won't work if the tether item is not in the inventory of the protector.

Feel free to use in modpacks.

Legacy:

(Forge 1.21.1)
Right-click the Item by the player, which is supposed to receive damage.
The linked tether item needs to be inside of the inventory of the player who should be protected.
If the protecting player has a tether item in the inventory, damage will not be transferred.

Shapeless crafting from 1x string.
Found in the creative tab Tools & Utilities.​

If you want the source code, just send me a message.

Feel free to use in modpacks.