Get an event when tags edited on Tag Manager.

Publisher.cs

using UnityEditor;
using UnityEditorInternal;

namespace Kanonji.TagManagerEvent
{
	public delegate void OnTagsChangedHandler(string[] tags);

	public class Publisher
	{
		public static event OnTagsChangedHandler OnTagsChanged;

		[InitializeOnLoadMethod]
		public static void InitEditorUpdateCallback()
		{
			int interval = 6;
			int nextTiming = (int) EditorApplication.timeSinceStartup;
			string tokenBefore = MakeToken(InternalEditorUtility.tags);

			EditorApplication.CallbackFunction function = () =>
			{
				if (EditorApplication.timeSinceStartup < nextTiming) return;
				nextTiming += interval;

				string token = MakeToken(InternalEditorUtility.tags);
				if (false == tokenBefore.Equals(token))
				{
					tokenBefore = token;
					OnTagsChanged(InternalEditorUtility.tags);
//					Debug.Log("InternalEditorUtility.tags changed");
				}
			};
			EditorApplication.update += function;
		}

		protected static string MakeToken(string[] tags)
		{
			return string.Join(",", tags);
		}
	}
}

Subscriber.cs

using UnityEngine;
using UnityEditor;

namespace Kanonji.TagManagerEvent
{
	public class Subscriber
	{
		[InitializeOnLoadMethod]
		public static void Subscribe()
		{
			Publisher.OnTagsChanged += Foo;
		}

		protected static void Foo(string[] tags)
		{
			Debug.Log(string.Join(",", tags));
		}
	}
}