/usr/lib/x86_64-linux-gnu/libmate-panel-applet-4.so is in libmate-panel-applet-dev 1.20.1-3ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libmate-panel-applet-4.so.1.0.1