AFAICT the file needs to be in the root of the GH Pages source directory, which is now a subdirectory.
This makes the repo root more approachable for newcomers; the README can be seen in GitHub immediately instead of after much scrolling.
All content that is reachable via links on zips.z.cash is moved into this directory, so that GitHub Pages can be configured to build the site from it. This includes non-generated files like CSS that are required for correct site presentation.