Get screen bounds with **rotated** ortho camera??

If your game is 2½D (i.e, 3D view, 2D gameplay) and you want to instantiate objects at screen edges on a plane, cast a ray from the edge of the viewport onto a plane you define. Then you will be able to instantiate objects on that plane, at viewport edges.

Otherwise I don't know what your distance that you talk about in your comments is. Distance to what?

Perspective camera

Orthographic camera

Here I cast a ray from screen edges on a plane at origo. I added the box to more clearly see where that plane would be. I am instantiating spheres here, ladies and gentlemen. It doesn't get better than that. The camera is rotated along all three axes.

Was this effect something like what you were looking for? You might want to move the objects slightly off camera so they don't pop into view the way they do here, but that is another sub problem to be solved on its own right.

 using UnityEngine;
 
 public class ScreenEdgeExample : MonoBehaviour
 {
     public GameObject prefab;
 
     void Start()
     {
         ScreenInstantiate.LeftEdge(prefab);
         ScreenInstantiate.RightEdge(prefab);
         ScreenInstantiate.TopEdge(prefab);
         ScreenInstantiate.BottomEdge(prefab);
     }
 }
 
 public static class ViewportPoint
 {
     public static readonly Vector2 Up = new Vector2(0.5f, 1.0f);
     public static readonly Vector2 Down = new Vector2(0.5f, 0.0f);
     public static readonly Vector2 Left = new Vector2(0.0f, 0.5f);
     public static readonly Vector2 Right = new Vector2(1.0f, 0.5f);
 }
 
 public static class ScreenInstantiate
 {
     public static GameObject LeftEdge(GameObject original)
     {
         return AtViewportPoint(original, ViewportPoint.Left);
     }
 
     public static GameObject RightEdge(GameObject original)
     {
         return AtViewportPoint(original, ViewportPoint.Right);
     }
 
     public static GameObject TopEdge(GameObject original)
     {
         return AtViewportPoint(original, ViewportPoint.Up);
     }
 
     public static GameObject BottomEdge(GameObject original)
     {
         return AtViewportPoint(original, ViewportPoint.Down);
     }
 
     public static GameObject AtViewportPoint(GameObject original, Vector2 viewportPoint)
     {
         // Plane is facing back and at (0, 0, 0).
         // Depending on your game, you may want to change this (to ground for top-down)
         // Consider promoting plane to a parameter.
         Plane plane = new Plane(-Vector3.forward, Vector3.zero);
         Ray ray = Camera.main.ViewportPointToRay(viewportPoint);
         float distance;
 
         if (plane.Raycast(ray, out distance))
         {
             GameObject clone = Object.Instantiate(original);
             clone.transform.position = ray.GetPoint(distance);
             return clone;
         }
         else
         {
             // Fallback solution...
             Debug.LogWarning("Failed to Instantiate prefab at viewport point.\n"+
                              "Defaulting to a normal Instantiate.");
             return Object.Instantiate(original);
         }
     }
 }