This is a custom LocalPlayer class that overrides the behaviour of the aspect ratio axis mode to work kinda like the "Maintain Minor Axis" mode, but it takes into account whatever Aspect Ratio you have set on the camera.
This overrides the Override Aspect Ratio Axis setting completely and does not require any .ini edits. Also, no stereo/VR/XR support, this is purely for flat 2D screens.
What it does is if (ScreenWidth < ScreenHeight * AspectRatio), then it will do the same behaviour as the AspectRatio_MaintainXFOV, making it so that your 16:9 (or whatever Aspect Ratio you set on the camera) image is maintained regardless if the window is taller/your phone is in portrait mode/you put the game window to the side to have focus on something else while you play it, while you still get the intended game experience.
When you're using an ultrawide screen (or you have a very wide window), you will get a wider horizontal field of view (like AspectRatio_MaintainYFOV), while keeping the vertical FOV exactly the same.