Enabling Unity's SpriteAtlasV2 only when entering play mode to prevent constant sprite packing.

SpriteAtlasAutoEnable.cs

using UnityEditor;

[InitializeOnLoad]
public static class SpriteAtlasAutoEnable
{
    private const string cEditorPrefSpriteAtlasAutoEnable = "SpriteAtlasAutoEnable.AutoEnable";
    private const string menuName = "Tools/SpriteAtlas AutoEnable/Enabled";

    public static bool AutoEnable
    {
        get => EditorPrefs.GetBool( cEditorPrefSpriteAtlasAutoEnable, true );
        set
        {
            EditorPrefs.SetBool( cEditorPrefSpriteAtlasAutoEnable, value );
            if( value )
                EditorSettings.spritePackerMode = EditorApplication.isPlayingOrWillChangePlaymode ? SpritePackerMode.SpriteAtlasV2 : SpritePackerMode.SpriteAtlasV2Build;
            
            Menu.SetChecked(menuName, value);
        }
    }

    static SpriteAtlasAutoEnable()
    {
        EditorApplication.playModeStateChanged += OnEnterPlayMode;
        if( AutoEnable )
            EditorSettings.spritePackerMode = EditorApplication.isPlayingOrWillChangePlaymode ? SpritePackerMode.SpriteAtlasV2 : SpritePackerMode.SpriteAtlasV2Build;
        
        Menu.SetChecked(menuName, AutoEnable);
    }

    private static void OnEnterPlayMode(PlayModeStateChange obj)
    {
        if( !AutoEnable )
            return;

        EditorSettings.spritePackerMode = obj switch
        {
            PlayModeStateChange.ExitingEditMode => SpritePackerMode.SpriteAtlasV2,
            PlayModeStateChange.ExitingPlayMode => SpritePackerMode.SpriteAtlasV2Build,
            _                                   => EditorSettings.spritePackerMode
        };
    }

    [MenuItem( menuName )]
    private static void ToggleEnabled()
    {
        AutoEnable = !AutoEnable;
        Menu.SetChecked(menuName, AutoEnable);
    }
}

For those like me that are annoyed by constant sprite packing since the switch to spriteatlasv2, here is a script that, when enabled, set the sprite packing mode to "Build Only" then switch to "Always" when entering play mode. This will mimics the old spriteatlas behavior by sprite packing only when entering play mode.