Is there an in-game list that shows all armor pieces and requirements to unlock them?
Solution 1:
You are right there are no lists for Armour unlocking. The only list you will get is in challenges under operations. So far you can get the complete set of K.V.A Assassin outfit, That is under Killer. There are also other Armour but it is shown under Weapons. That Armour is full Diamond.
Then here is a list of everything BUT ARMOUR!