Where are the Server Backup API files wsbapp.h, wsbapp.idl?

The MSDN Library article on Windows Server Backup says that the Windows Server Backup API files wsbapp.h and wsbapp.idl can be found in the Windows SDK Update for Server 2008, but no such update exists. These files did not ship in the Windows SDK for Server 2008 and .NET Framework 3.5. You can find this content on the Microsoft Download center - https://www.microsoft.com/downloads/details.aspx?FamilyID=A88B8404-0228-4F23-AAED-F04FFAD3B89C&displaylang=en.

The Windows Server Backup feature automatically backs up and restores data for an application that contains a Volume Shadow Copy Service (VSS) writer.