/usr/lib/x86_64-linux-gnu/vala-panel/applets/tasklist-xfce.plugin is in vala-panel-plugins-wnck 0.3.65-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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Module=tasklist-xfce
Name=Tasklist Xfce
Description[ru]=Показывает открытые приложения
Description=Shows opened windows.
X-ValaPanel-Expandable=true
X-ValaPanel-Configurable=true
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