Assumption about the shape of your projection tend to creep into the weirdest places in complex renderers like the one in UE over time. I am willing to bet that

Assumption about the shape of your projection tend to creep into the weirdest places in complex renderers like the one in UE over time. I am willing to bet that a lot of the screen space lighting and post processing shaders had assumptions like that baked in. This can make adding a different projection type a heavy lift, even though nothing about it is magic.