The three items mentioned in the original post (Combat knife, Santa Poring, and Deviling hats) do not stack with Forsaken King's shield or Friggs King's Shield. Meaning: If you are wearing one of these two shields and equip one of the above mentioned items you will not get further damage reductions from said items.
Sacred Wings and all other damage reducing items should still work as they did before.
If you want to try to get more reductions you can use a class specific Elite Shield or Friggs version of the elite shield. The headgears and combat knife still stack with those.
At least this is how I understand it. If I'm wrong feel free to correct me.