Off-Camera view Object tracker

Getting the line between the player and your target is easy. But then to put the arrow on that line requires some college algebra: the slope-intercept formula. With that formula you can pick an X pos (the edge of the screen) to get the Y pos of the line at that point.

     public GameObject player;
     public GameObject target;
     public Image arrowPointerUI;
 
     public void Update()
     {
         float x = 0f;
         float y = 0f;
 
         Vector2 A = Camera.main.WorldToScreenPoint(player.transform.position);
         Vector2 B = Camera.main.WorldToScreenPoint(target.transform.position);
 
         // slope
         var m = (B.y - A.y) / (B.x - A.x);
         // y-intercept
         var b = A.y + (-m * A.x);
 
         // x and y formulas reference
         // y = (m * x) + b;
         // x = (y - b) / m;
 
         Vector2 offset = arrowPointerUI.rectTransform.sizeDelta / 2;
         Rect scrRect = new Rect(offset, new Vector2(Screen.width - offset.x, Screen.height - offset.y));
 
         // clamp target x pos to screen to help find y
         x = Mathf.Clamp(B.x, scrRect.xMin, scrRect.xMax);
         y = (m * x) + b;
 
         // if y is off screen clamp it, then find a better x
         if (y < scrRect.yMin || y > scrRect.yMax) {
             y = Mathf.Clamp(y, scrRect.yMin, scrRect.yMax);
             x = (y - b) / m;
         }
 
         // arrow position
         Vector2 C = new Vector2(x, y);
 
         // this checks if target is on screen or not
         if (Vector2.Distance(B, C) > offset.x) {
             // target is far from arrow, so it is off screen
             // show arrow and position it
             arrowPointerUI.enabled = true;
             arrowPointerUI.transform.position = C;
 
             // rotate arrow to point from player to target
             float angle = Mathf.Atan2(B.y - A.y, B.x - A.x) * Mathf.Rad2Deg;
             arrowPointerUI.transform.rotation = Quaternion.AngleAxis(angle, Vector3.forward);
 
         } else {
             // target is close to arrow, so it is on screen
             // hide arrow
             arrowPointerUI.enabled = false;
         }
     }
 
     private void OnDrawGizmos()
     {
         // Draw line between player and target for debugging
         Vector2 A = player.transform.position;
         Vector2 B = target.transform.position;
         Gizmos.DrawLine(A, B);
     }

This assumes a top-down view with the arrow being an UI Image and that it is square. This works fine as long as the player remains inside the scrRect bounds that was set up. If the player leaves those bounds, you can set up a check to hide the arrow.