Is there a setting to default the Editor Game View to a specific aspect ratio?

I looked further into it and I found this link that might help you: https://answers.unity.com/questions/956123/add-and-select-game-view-resolution.html

I also tried to work out a simpler solution, that does not require actual Editor scripts. This solution grew quite big, since it needs a lot of commentary because it is not so straight forward what is happening under the hood. I attached my script below.

What my script essentially does is, it looks for the current aspect ratio of the game view, and if it is not set to 9:16, a Debug Message informs the programmer that they have to set the game view to 9:16.

It seems that Unity has some rounding issues when it comes to calculating the aspect ratio of the game view. So instead of checking if the current ratio is exactly 9:16 (=0.5625), I rather check if it is close to that number.

I can imagine that Unity returns different results on different computers, that's why I also added a debug mode, that allows to see what ratio Unity finds on your machine. I hope that my code or the link delivers a valid solution for you.

 using System.Collections.Generic;
 using UnityEngine;
 using System.Linq;
 
 // Description:
 // Attach this script to a GameObject in your Project
 // This script will ensure that the game view window is always set to an aspect ratio of 9:16
 // It checks the current aspect ratio and informs the programmer with a debug message if the aspect ratio is not set to 9:16
 //
 //
 //
 // Summary:
 // When the game view ratio is set to 9:16, it is expected that the calculation '(float)Screen.width / (float)Screen.height' returns 9:16 = 0.5625
 // This is not always the case (I am unsure why). 
 // When you set your game view ratio to 9:16 and you squeeze the view horizontally (left/right), to the point that Unity draws letter boxes (gray bars
 // on top and bottom to maintain a ratio of 9:16), then the calculation returns values that are close to 0.5625, but not exact.
 // In my tests it returns values between 0.5611111 and 0.5638298
 // That is why I check for a ratio range in Update(), instead of just looking for (float)Screen.width / (float)Screen.height == 9f/16f
 //
 // If you want to find out what values your Editor returns for (float)Screen.width / (float)Screen.height, first set your game view window to 9:16.
 // Enable the debugMode bool in the inspector and play around with the width of your game view 
 // Slowly stretch the windows enough so that Unity draws Pillar boxes (gray bars left and right), and squeeze it enough to force Unity to draw letter boxes 
 // (gray bars on top and bottom)
 // After you have done this, rightclick on the script in the inspector and select "Print MinMax" to print the smallest and biggest ratio that Unity detected
 // Use these numbers to set your min max values in the if-statment.
 // For me that is 0.5611111 and 0.5638298
 // So to make sure I dont trigger that if-statmen accidentally, I set my range from 0.560 to 0.564
 
 [ExecuteInEditMode]
 public class AspectRatioChecker : MonoBehaviour
 {
     [Tooltip("Debug Mode: Use this to determent a better ratio range for your game view (Read Summary inside this script for more information).")]
     [SerializeField] private bool debugMode = false;
 
     private List<float> ratios = new List<float>();
 
 
     private void Update()
     {
         // Only do this in Editor and while not in Playmode
         if (Application.isPlaying) return;
 
         Debug.Log("Is Running!");
 
         float ratio = (float)Screen.width / (float)Screen.height;
 
         // Check if Aspect ratio is 9:16 (or very close to it)
         if (0.560 <= ratio 
             && ratio <= 0.564)
         {
             // We are in desired Range (very close to 9:16)
         }
         else
         {
             // Not in desired Range
             Debug.Log($"Wrong aspect ratio deteced. Please set the aspect ratio of the game view to 9:16");
         }
 
 
         // Debug Mode
         if (debugMode)
         {
             // To prevent the list from growing too big
             if (ratios.Count < 1000)
             {
                 ratios.Add(ratio);
                 Debug.Log($"Ratio: {ratio} " + '\n' +
                     $"List length: {ratios.Count}/1.000");
             }
             else
             {
                 Debug.Log($"Ratio List already holds 1000 values. Please clear the List or disable DebugMode.");
             }
         }
         else
         {
             ratios.Clear();
         }
     }
 
     [ContextMenu("Print MinMax")]
     private void PrintMinMax()
     {
         Debug.Log($"##### Elements in List: {ratios.Count} #####");
         Debug.Log($"Min: {ratios.AsQueryable().Min()}");
         Debug.Log($"Max: {ratios.AsQueryable().Max()}");
     }
 
 
     [ContextMenu("Empty List")]
     private void EmptyList()
     {
         Debug.Log($"List cleared!");
         ratios.Clear();
     }
 }

In Game-View one centimeter below there is the word "Display 1". Right of that there is the word "Free Aspect". Click on it and select Aspect Ratio