A monobehaviour and extension function combination to receive a callback when a gameobject is destroyed

OnDestroyCallback.cs

using System;
using UnityEngine;

public class OnDestroyCallback : MonoBehaviour {

    Action onDestroy;

    public static void AddOnDestroyCallback(GameObject gameObject, Action callback) {
        OnDestroyCallback onDestroyCallback = gameObject.GetComponent<OnDestroyCallback>();
        if (!onDestroyCallback) {
            onDestroyCallback = gameObject.AddComponent<OnDestroyCallback>();
            onDestroyCallback.hideFlags = HideFlags.HideAndDontSave;
        }
        onDestroyCallback.onDestroy += callback;
    }

    private void OnDestroy() {
        if (onDestroy != null) {
            onDestroy();
        }
    }
}

public static class OnDestroyCallbackExtensions {
    public static void AddOnDestroyCallback(this GameObject gameObject, Action callback) {
        OnDestroyCallback.AddOnDestroyCallback(gameObject, callback);
    }
}