AspectRatio.cs

using UnityEngine;

/// <summary>
/// Aspect ratio controller. Must be attached to the main camera.
/// </summary>
[RequireComponent(typeof(Camera))]
public class AspectRatio : MonoBehaviour
{
    [Tooltip("The idea ratio that you want the world rendered at")]
    public float perfectRatio = 16f / 9f;

    private void Start()
    {
        // Grab the camera component from the game object
        var camera = GetComponent<Camera>();

        // Calculate the actual screen ratio
        float aspectRatio = Screen.width / (float)Screen.height;

        // Early out if the aspect ratio matches the perfect ratio
        if (Mathf.Abs(aspectRatio - perfectRatio) < float.Epsilon)
            return;

        // Calculate the viewport height based on the current height and the perfect ratio
        var viewportHeight = (Screen.width / perfectRatio) / Screen.height;

        // Calculate the top-most position for the viewport
        var yPos = (1f - viewportHeight) / 2f;

        // Set the viewport
        camera.rect = new Rect(0, yPos, 1f, viewportHeight);
    }
}