Since OnLevelWasLoaded is deprecated (in 5.4.0b15) What should be use instead?

Since a reliable coding example hasn't come up in the first few Google searches, I thought I'd post the new suggested implementation here.

The old way:

 void OnLevelWasLoaded (int level)
 {
 //Do Something
 }

The new way:

     using UnityEngine.SceneManagement;
             void OnEnable()
             {
              //Tell our 'OnLevelFinishedLoading' function to start listening for a scene change as soon as this script is enabled.
                 SceneManager.sceneLoaded += OnLevelFinishedLoading;
             }
         
             void OnDisable()
             {
             //Tell our 'OnLevelFinishedLoading' function to stop listening for a scene change as soon as this script is disabled. Remember to always have an unsubscription for every delegate you subscribe to!
                 SceneManager.sceneLoaded -= OnLevelFinishedLoading;
             }
         
             void OnLevelFinishedLoading(Scene scene, LoadSceneMode mode)
             {
                 Debug.Log("Level Loaded");
                 Debug.Log(scene.name);
                 Debug.Log(mode);
             }

Note that 'OnLevelFinishedLoading' is a name that I've made up. You can name your method whatever you like.

What you're seeing in the OnEnable and OnDisable functions are delegate subscriptions. This simply means that we are setting a function of our choice (in this case, 'OnLevelFinishedLoading') to listen to the SceneManager for a level change.

Also note, that since this delegate has two parameters (Scene and SceneMode), that you must include those two parameters as well - even if you don't plan on using that information in your function.