UITransitionView and UILayoutContainerView

Question

They are both internal views used by Apple and not published in the SDK.

It's difficult to say exactly what Apple will or won't do however, adding a view to an undocumented view you retrieve from a superView message is not contentious. What you should not do (or be careful if you do do) is make assumptions about the view you are adding to. Specifically its class but even basic things like the fact that it even exists.

What are you trying to do? There may be a simpler way - like adding your view directly to the app's UIWindow.