Tastenabfrage im EditMode 100x pro Sekunde ?

Sorry but you've mixed completely different and independent concepts here. The Event class contains information about the current raised event. It only has meaning inside of an Event callback that is called by Unity. The Event class does not allow you to poll anything. Furthermore you named your method "OnSceneGUI" which is a callback from Unity which is called automatically by unity. You never ever want to call Unity callbacks manually through any other means. Registering it to the generic update delegate makes no sense.

It's not clear what you actually want to do. The whole Unity editor is completely event based. It this is part of a custom editor / inspector you can simply override the RequiresConstantRepaint method and return true.

 public override bool RequiresConstantRepaint()
 {
     return true;
 }

This will make the inspector to redraw itself constantly. However you should be careful with such tools. To manually cause a redraw of the sceneview(s) you usually use SceneView.RepaintAll.

Regardless of all that you can always just set a boolean variable to true while a key is held down.

 bool isAltDown = false;
 
 void OnSceneGUI()
 {
     Event e = Event.current;
     if (e.isKey)
         isAltDown = e.alt;
 }

Though as I said it highly depends on whats the point of your code that should run 100 times a second. For example in my UVViewer editor window I redraw the sceneview constantly when the CTRL key is held down because I draw things in my editor window as well as the scene view and this needs to be updated.

So just to make that crystal clear: Just because you subscribed the OnSceneGUI method to the update delegate, doesn't mean you can access the Event class (or Event.current to be more precise). The Event class is not magically bound to a certain method but is explicitly set by Unity before it calls this callback. If you call this callback from somewhere else, there is no active event.

Just for reference, my IMGUI crash course.