Is it still impossible for BHVR to make Never Stop Slashing mask work as intended by the designer?

So, to make an animated texture like that, it could be done in several ways depending on how you want the final product to look. JUST changing from one static expression to another, they could do that the texture and UV Map. Essentially, they would put all expressions onto a single texture for the mask, and have the game offset the texture based on what it is supposed to be showing. The UV Map itself would only be designed to use one of these squares, and the offset would make the mask treat every square the same.

Now, if they want it animated, it's a bit different. They would have to do the same thing but with a much larger texture, and have the texture play certain squares in specific orders, then reset to the start of that animation string, and then jump to a different part of the texture for a change. Essentially, it would be a GIF on the face. At this point, it would be easier to tell the model to swap facial textures based on what the expression is.

The third, and probably most jank option, is to essentially have movies playing on the mask. This would, well, it'd be jank. Scrolling textures, which a lot of killers use, are easy enough to do. GIFs and Movie files played on the surface of a character, however, is much harder. I don't even know how much the GIF and Movie functions are supported by UE. I know they are possible in other engines, though they are often not used because they tend to look a bit meh.