void09 [1] filed a deletion request for xfce4-taskbar-plugin [2]:

"Since the 4.4 version of the Xfce, the taskbar plugin is shipped with the panel. So there is no need to install any other stuff."

Also the source url is gone.

[1] https://aur.archlinux.org/account/void09/ [2] https://aur.archlinux.org/pkgbase/xfce4-taskbar-plugin/