Simulate Unreal engine in Unity (just shows the "LIGHTING NEEDS TO BE REBUILT" in the top left corner of the viewport.) based on an original idea by Kenney (https://github.com/KenneyNL/Unreal-Simulator-for-Unity) but this time, it is just a 23 lines editor script, and not a full package with prefabs, assets, etc...

UnrealSimulatorEditor.cs

using UnityEngine;
using UnityEditor;

[InitializeOnLoad]
public class UnrealSimulator : Editor
{
	private static GUIStyle style	= null;
	private const string	text	= "LIGHTING NEEDS TO BE REBUILT (4 unbuilt object(s))";
	private const string	subtext = "'DisableAllScreenMessages' to suppress";

	static UnrealSimulator()
	{
		SceneView.duringSceneGui += (SceneView sceneview) =>
		{
			style ??= new GUIStyle(GUI.skin.label) { richText = true, fontSize = 13, font = Font.CreateDynamicFontFromOSFont("Verdana", 13) };

			Handles.BeginGUI();
			GUI.Label(new Rect(new Vector2(21, 21), new Vector2(Screen.width, 20)), "<color=#00000080>" + text + "</color>", style);
			GUI.Label(new Rect(new Vector2(20, 20), new Vector2(Screen.width, 20)), "<color=#ff0000ff>" + text + "</color>", style);
			GUI.Label(new Rect(new Vector2(60, 40), new Vector2(Screen.width, 20)), "<color=#8c8c8cff>" + subtext + "</color>", style);
			Handles.EndGUI();
		};
	}
}