Camera horizontal distortion?

Cameras use a projection matrix to map things to the screen for rendering. As part of this, things like field of view, clipping planes and aspect ratio are taken into account - separate from resolution. As such, we can override the camera's projection matrix using a custom aspect ratio without actually modifying the screen's aspect ratio;

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class HorizontalDistortion : MonoBehaviour
 {
     private Camera cam;
 
     //This is in relation to the rendering aspect ratio, clamped to a minimum to stop errors
     [Min(0.01f)]
     public float aspectMultiplier = 1f;
 
     //Before the camera culls the scene
     private void OnPreCull ()
     {
         //Get the camera if we don't already have it
         if (!cam)
             cam = GetComponent<Camera>();
 
         //Override the projection matrix, only adjusting aspect ratio
         cam.projectionMatrix = Matrix4x4.Perspective (cam.fieldOfView, cam.aspect / aspectMultiplier, cam.nearClipPlane, cam.farClipPlane);
     }
 
     private void OnDisable ()
     {
         //Reset the projection matrix when we are done
         if (cam)
             cam.ResetProjectionMatrix ();
     }
 }