AFAIU xmlrpclib.MultiCall implements the technique described on the broken link, and the examples look pretty clear to me, so the user doesn't have to follow the link to understand how to use it.
I think it's good to keep a link around, in case the user wants to understand better how it works, but it could also be in a footnote.

This is what I would do:
  1) write something like "The xmlrpc.MultiCall object provides a way to  encapsulate multiple calls to a remote server into a single request¹."
  2) in the footnote write something like "[1]: This approach has been first presented in `a discussion on xmlrpc.com <link-to-web.archive.org>`_."

Possibly you could leave an rst comment explaining that the original link is down and that we are using the web.archive.org one until the page is back up.
FWIW http://www.xmlrpc.com/discuss/topics seems to work, and there are a few discussions from last month, but the links are broken.