How can i code to delete unused bindings in PlayableDirector?

This may have changed in later versions I think. In Unity 2021.2.16 / Timeline 1.6.4 at least there's a little minus sign at the very bottom-right of the Bindings field. Highlight the unused binding (or entire unused Timeline), and hit the minus to remove the ones not used.

It's hard to delete the unused bindings...since it stored in a property named 'm_SceneBindings' which cannot be read/written directly. A workaround to remove the unused bindings is to create a new Playable Director and copy the bindings from the old one.

Sample code:

 private static void PlayableDirectorRebinding(PlayableDirector playableDirector)
 {
     if (playableDirector != null)
     {
         var timeline = playableDirector.playableAsset as TimelineAsset;
         if (timeline != null)
         {
             var bindings = new Dictionary<Object, Object>();
             foreach (var playableBinding in timeline.outputs)
             {
                 bindings.Add(playableBinding.sourceObject, playableDirector.GetGenericBinding(playableBinding.sourceObject));
             }
             var timelineGameObject = playableDirector.gameObject;
             GameObject.DestroyImmediate(playableDirector);
             playableDirector = timelineGameObject.AddComponent<PlayableDirector>();
             foreach (var entry in bindings)
             {
                 playableDirector.SetGenericBinding(entry.Key, entry.Value);
             }
         }
     }
 }

Reference form here: https://blog.csdn.net/cai612781/article/details/99947344