Mike M, an engineer on the Windows Camera team at Microsoft, explained Microsoft’s reasons for removing this feature in a thread on the Microsoft forums. If you installed the Anniversary Update when it was first released, there’s a good chance you can’t downgrade any more. Unfortunately, with the Anniversary Update, Microsoft quietly changed the downgrade period from 30 days down to 10 days.
If you have this problem, you can downgrade from the Anniversary Update to the previous version of Windows.