This directory used to contain a shell completion function for zsh.
That script has been unmaintained since 2005, though.  Zsh itself does
ship a _bzr completion function, and has done so for quite some time
now.  So we suggest you use the version installed with your zsh
instead of the version that used to be located in this directory.

Note that the shell-complete builtin of bzr does provide completion
information in a style suitable for zsh.  There are some completion
scripts for zsh in the wild that make use of this feature, although
none of these scripts is officially supported by the bzr developers.

Also note that recent versions of bzr (starting with the 2.3 series)
do include a bash_completion plugin. It should be possible to improve
this in such a way that it generates code for zsh instead of bash.  So
if you want better zsh completion, this might be the place to start.

References:
https://bugs.launchpad.net/bzr/+bug/560030
http://zsh.git.sourceforge.net/git/gitweb.cgi?p=zsh/zsh;a=history;f=Completion/Unix/Command/_bzr