I just checked, and yes, they really are gone. The only PC-based Nook app currently available on that page is the Windows 8 version, which won’t help very many people.
As the article notes, you can still read Nook books on your PC using the browser-based reader, but that doesn’t allow you to download books for archival (or DRM-removal) purposes. It’s also not helpful for indie and self-published authors who want to check the formatting of their books for Nook (without actually needing to buy a Nook). I found use of the Nook for PC program to be vital when I was fixing the formatting of my non-fiction book. Kindle handled it well. Nook gave me fits until I figured out how to properly format lists.
Apparently, B&N Customer service isn’t giving any reasons. We don’t know if it’s temporary or permanent, although I’d assume permanent until we learn otherwise.
Fortunately, you can still get them, using these links
If you buy Nook books and want a backup or the ability to remove DRM on them, I’d suggest downloading them now, while you still can. I grabbed a copy of the install files, just to have them.