[superfrog]

If anybody cares, it turns out that Google Drive had pushed a security update to shared folders last fall. Now you need a resource key to get access to a shared folder, even if the folder is set to public access (like this one was). Anybody who had previously viewed the folder was exempted from this.

I had gotten a new link that I put in my comment above, which worked because it has the resource key included. Then when I went to put that link in the OP, I deleted everything after the question mark in the link (as I often do, usually Google just uses that for tracking purposes). So I had just gotten too clever for my own good, and now the link should work going forward.