Mirrors not allowing Directory Listings

Hi,

I frequently take a manual look into package directories, for example by
directly typing URLs like ftp2.de.debian.org/debian/pool/main/a/aide/
into my browser.

Incidentally, ftp2.de.debian.org doesn't allow directory listings,
returning a 403 for this request.

I would like to know whether this is an acceptable configuration for an
ftpX.COUNTRY.debian.org mirror, and if yes, can Debian please consider
adding a guideline that directory listings should be allowed for a
public archive of Debian?

Or does such a guideline already exist?

Greetings
Marc