I found one way to get the original 1607 ISO from a Microsoft server.
First, you have to convince Microsoft that you are not running Windows. So use your favorite method to change your user-agent string to something else, or use not Windows. I used:
Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_4) AppleWebKit/600.7.12 (KHTML, like Gecko) Version/8.0.7 Safari/600.7.12
to be Safari on OSX. Then visit
https://www.microsoft.com/en-us/software-download/windows10ISO
(On windows this will direct you to the Media Creation Tool, which builds you a custom ISO not suitable for use with deployment tools like Microsoft Deployment Toolkit; but for non-windows users, you can get a real link to the original ISO.)
Pick the edition (I wanted Windows 10 Anniversary Update > Windows 10) and the language (English), and you'll be given custom links to the ISO after your request is "validated".
This doesn't help if you want the original 1511 en-US ISO (also not available on MSDN), and is obviously not as ideal as being able to go to MSDN to get original vesions of every Windows release.