SelectionOrder.cs

// using System.Collections.Generic;
// using UnityEngine;
// using UnityEditor;

        // for ordered selection
        HashSet<GameObject> selectionSet = new HashSet<GameObject>();
        HashSet<GameObject> newSet = new HashSet<GameObject>();
        HashSet<GameObject> deleteSet = new HashSet<GameObject>();
        List<GameObject> selectionOrdered = new List<GameObject>();
 
        private void OnSelectionChange()
        {
            newSet.Clear();
            newSet.UnionWith(Selection.gameObjects);
 
            deleteSet.Clear();
            deleteSet.UnionWith(selectionSet);
            deleteSet.ExceptWith(newSet);
 
            foreach (var g in deleteSet)
            {
                selectionSet.Remove(g);
                selectionOrdered.Remove(g);
            }
 
            newSet.ExceptWith(selectionSet);
            foreach (var g in newSet)
            {
                selectionSet.Add(g);
                selectionOrdered.Add(g);
            }
 
            for (var i = 0; i < selectionOrdered.Count; ++i)
            {
                Debug.Log(i + " " + selectionOrdered[i].name);
            }
        }

for getting the order of selections in Unity's hierarchy.