/usr/lib/unity-2d/plugins/panel/libpanelplugin-legacytray.so is in unity-2d-panel 5.12.0-0ubuntu1.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/unity-2d/plugins/panel/libpanelplugin-legacytray.so.0.0