The list of all webservers used to fit on a single page, which might be a good starting point. However, at this point I believe anything still functioning might fit your definition of a "mirror" as few things still speak HTTP/0.9.
dated 30 Sept 1992, which is pretty old as far as the web goes.
I would agree that an archive ala archive.org should be disqualified, because few people would go to that site as the original. To me, Sunet is a little more grey, because while it is an archive/mirror site, many people used it and linked to it as if it were the primary source and not just an archive site. Before the web was indexed and search engines became popular, large mirror sites were the easiest way to access a variety of content, all in one place.
(Meaning the the current maintainers could fix this by doing a single "ln -s" command, ehrm. @ acc.umu.se.)
Anyway, a previous webmaster of this site replied another time it was posted here, https://news.ycombinator.com/item?id=23709016 if anyone's curious.