Is there a way to tell in Unity iOS when onDidReceiveMemoryWarning is received?

I found the answer for anyone interested.

In the AppController.mm

Look for the function:

 - (void) applicationDidReceiveMemoryWarning:(UIApplication*)application
 {
  printf_console("WARNING -> applicationDidReceiveMemoryWarning()\n");
 }

This is the function that is called when your app receives a low memory warning.

Next you must add a UnitySendMessage to your Unity code, so that you can receive the low memory warning message in your Unity code.

So, you must modify the applicationDidReceiveMemoryWarning function by writing this code:

 - (void) applicationDidReceiveMemoryWarning:(UIApplication*)application
 {
  printf_console("WARNING -> applicationDidReceiveMemoryWarning()\n");

     // This is your added call to send a message to your Unity code.
     // The first parameter is the name of the class you'll be using to receive the
     // low memory warning message. Mine is MemoryManager, yours can be whatever
     // you name your class.
     // The second parameter is the name of the method/function within your class
     // that you will be using to receive the low memory warning and handle it 
     // appropriately however you see fit. Mine is ReceivedMemoryWarning, yours can
     // be whatever you like.
     // Don't forget that your function/method within your Unity C#/Javascript/etc
     // code, must be a void return value with a string parameter.
     // E.g. 
     // public void ReceivedMemoryWarning(string message)
     // The third parameter can be a null string, or whatever string you want your 
     // message that is sent to say.
 
     UnitySendMessage("MemoryManager", "ReceivedMemoryWarning","");
 }

After you add this message call to the AppController.mm file, in your Unity code, add a new file for the class that will be receiving the message. Mine happens to be in C#, but it can be any file type. Mine is MemoyManager.cs.

Then add the code:

 using UnityEngine;
 using System.Collections;
 
 public class MemoryManager : MonoBehaviour
 {
  //Function is called when a lowMemoryWarning is received
  public void ReceivedMemoryWarning(string message)
  {
  Debug.Log("Memory Manager RECEIVED LOW MEMORY WARNING!");
  }
 }

This class now receives the message sent from the AppController.mm, and within your own function, mine is ReceivedMemoryWarning, handle how you deal with, free up memory within your game/project.

Also, it's worth mentioning if you want to modify the AppController.mm and have it update automatically in xCode every time, first create a folder in your Assets folder within the Project hierarchy window named "iOS" under the Plugins folder, so the hierarchy directory is Assets/Plugins/iOS. Then drag the AppController.mm file into that folder with your modified code, and Unity will automatically update your changes when it builds to xCode.

I hope that helps for everyone who is looking to do this, whether you're a programmer or not.