public Geometry PreviousSketch {get;}

// SketchModified event is fired by 
//  - COTS construction tools (except annotation, dimension geometry types), 
//  - Edit Vertices, Reshape, Align Features
//  - 3rd party tools with FireSketchEvents = true


//Subscribe the sketch modified event
//ArcGIS.Desktop.Mapping.Events.SketchModifiedEvent.Subscribe(OnSketchModified);

private void OnSketchModified(ArcGIS.Desktop.Mapping.Events.SketchModifiedEventArgs args)
{
  // if not an undo operation
  if (!args.IsUndo)
  {
    // what was the sketch before the change?
    var prevSketch = args.PreviousSketch;
    // what is the current sketch?
    var currentSketch = args.CurrentSketch;
    if (currentSketch is Polyline polyline)
    {
      // Examine the current (last) vertex in the line sketch
      var lastSketchPoint = polyline.Points.Last();

      // do something with the last point
    }
  }
}