Microsoft documents ntdll as subject to change and makes no promises not to break it. When I was at MS it was advised not to use it for things that ship outside

Microsoft documents ntdll as subject to change and makes no promises not to break it. When I was at MS it was advised not to use it for things that ship outside Windows. They had automated scripts to flag such a use. In practice it is pretty stable, and many use it. Many things cannot be changed without causing massive internal refactors and app compat issues. But it is regarded as internal.