hkp://keyring.debian.org used to return HTTP 406 when a user tried to do a gpg
--search (searching is not implemented in that keyserver). I pointed them at
the old HKP I-D, and they updated it to now return 501 when a search is attempted:

https://rt.debian.org/Ticket/Display.html?id=2220

(guest RT login is currently u:guest/p:readonly, according to
https://wiki.debian.org/rt.debian.org)

But gpg still does not report that the keyserver doesn't support the particular
query: http://bugs.debian.org/574915 -- instead, it tells the user that no key
was found. This seems misleading. And we ought to be able to do the Right
Thing, especially for keyserver operators/implementors who are willing to modify
their tool to report "unimplemented" properly.

#651 might also be relevant here.