I'm one of the admins of ftp.halifax.rwth-aachen.de, which currently
serves as ftp2.de.debian.org.
In addition to your question (I'm also interested in guidelines), here's
a bit of background information:
We disabled directory listing for the pool/ subdirectory, mostly because
it contains a lot of files. This might cause higher load, especially if
someone decides to crawl the directory. Furthermore, which is due to our
(mis?)configuration, directory indexes are not cached, but instead
generated by a slow backend server.
Some people decided to download whole distributions via HTTP(s),
although we offer rsync access. Disallowing directory listing helps to
discourage this.