Even though BitBucket service warned everyone quite in advance that it would stop hosting Mercurial repositories, it seems that in many months no-one bothered to convert XEmacs repository to Git.
The instructions at
xemacs.org are but obsolete now:
The list of repositories is now empty:
More, the old CVS repository (see http://xemacs.sourceforge.net/Develop/cvsaccess.html) is unavailable, either, since
alioth.debian.org was shut down more than two years ago.
Update: there's also a recent topic on Reddit: Archive of Bitbucket.org Mercurial repositories.