If you are an IT professional or developer with a paid Visual Studio (formerly MSDN) subscription, you can access the archived builds. Log in via the Visual Studio Subscriptions Portal.
This free open-source tool fetches direct ISO links from Microsoft's official servers (still hosting old versions). windows 10 1607 iso file download
Here is the information regarding the Windows 10 Version 1607 (Anniversary Update) ISO file download. If you are an IT professional or developer