This plugin is now shipped with xfce4-panel >= 4.4.0.
Maybe you are looking for https://git.xfce.org/panel-plugins/xfce4-taskbar-plugin/tree/README
Written by Andre Lerche, this is a taskbar plugin for the panel. The size of the taskbar can be changed via the properties dialog. It includes an “expand the panel to fill the screen” feature.
This plugin works as any taskbar, showing you the running applications. You can show, hide or close any application on right clicking on his icon on the plugin. He also have the ability to group tasks from the same application and can be expanded in order to take all the available space on the screen.
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
The only dependency is the Xfce4 panel (version >= 4.2beta1) itself. Nothing else is required.