There appear to be two main categories of restriction based on two specific elements: The first is guns, or gun-related elements. If skin has something like a pistol in a holster attached to it ...

Following leaks of weapon attachment systems, new weapon skins for existing guns have been shown off. Leaked by popular dataminer IronS1ghts, new files added to the game in the latest update ...

Any skin that innately has some sort of gun attached to it, usually something like a pistol in a holster, is restricted. In my own collection, that includes Lara Croft, Black Widow, Cable ...