xsu dropped from unstable. Use gksu.
Package: xsu
The xsu package was renamed upstream to gnomesu around April 2003 but
has since stopped development (see http://xsu.sourceforge.net for more
info.)
This package was removed from the unstable archives on
2003-01-28 to make way for gnomesu (bug# 177789) and then gnomesu was
removed from the testing and unstable on 2004-01-12 (bug# 223878) due
to lack of upstream development and because gksu provides a similar
service.
Since testing tracks unstable the package is no longer in testing as well.