For example try to move your rifle up or down when your arm is rested on your upper thigh. You will find it harder than moving it to the sides. That's what is represented by the oval aperture.

I'd like to keep that functionality in 1.13.

Also I don't like the idea of showing something to the player (always a circle) and calculating something different for shooting (the oval) afterwards.

Of course you can do whatever you want in your experimental project. I'm merely presenting my opinion here.