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.
Microsoft only offers the current ISO of a given feature release. If you want an older version use a non-official Microsoft source. – Ramhound – 2017-08-01T23:13:18.827
"Can anyone offer advice on how to get Microsoft to make it available once again?" - We as end users cannot force Microsoft to do anything – Ramhound – 2017-08-01T23:15:06.603
@Ramhound - this is not true. They currently offer original 1703 and updated 1703. And they have a link to original 1511 (that doesn't work). Further, it is only the en-us version that has the updates slipstreamed - e.g., the Bulgarian version does not have the updates slipstreamed. – aggieNick02 – 2017-08-02T15:02:34.613
@Ramhound - I did not mean this super literally. Of course Microsoft is the only one that can publish something on MSDN. Rather, is there a good place to direct feedback or get Microsoft's attention in the hopes that they see/understand the feedback and possibly act on it? – aggieNick02 – 2017-08-02T15:04:26.323
Feedback Application; Otherwise out of scope here at Superuser on how and where to provide feedback to Microsoft – Ramhound – 2017-08-02T23:23:40.417