/usr/share/squirrelmail/plugins/ is in squirrelmail-compatibility 2.0.16-1.
This file is owned by root:root, with mode 0o755.
..
/usr/share/squirrelmail/plugins/compatibility/
/usr/share/squirrelmail/plugins/compatibility/functions.php
/usr/share/squirrelmail/plugins/compatibility/includes/
/usr/share/squirrelmail/plugins/compatibility/includes/1.2.10/
/usr/share/squirrelmail/plugins/compatibility/includes/1.2.10/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.2.11/
/usr/share/squirrelmail/plugins/compatibility/includes/1.2.11/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.2.7/
/usr/share/squirrelmail/plugins/compatibility/includes/1.2.7/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.2.8/
/usr/share/squirrelmail/plugins/compatibility/includes/1.2.8/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.2.9/
/usr/share/squirrelmail/plugins/compatibility/includes/1.2.9/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.0/
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.0/global.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.0/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.1/
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.1/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.10/
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.10/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.11/
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.11/global.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.11/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.12/
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.12/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.13/
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.13/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.14/
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.14/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.15/
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.15/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.16/
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.16/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.17/
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.17/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.18/
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.18/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.19/
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.19/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.2/
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.2/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.20/
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.20/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.3/
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.3/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.4/
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.4/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.5/
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.5/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.6/
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.6/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.7/
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.7/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.8/
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.8/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.9/
/usr/share/squirrelmail/plugins/compatibility/includes/1.4.9/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.5.0/
/usr/share/squirrelmail/plugins/compatibility/includes/1.5.0/global.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.5.0/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.5.1/
/usr/share/squirrelmail/plugins/compatibility/includes/1.5.1/global.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.5.1/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.5.2/
/usr/share/squirrelmail/plugins/compatibility/includes/1.5.2/global.php
/usr/share/squirrelmail/plugins/compatibility/includes/1.5.2/index.php
/usr/share/squirrelmail/plugins/compatibility/includes/index.php
/usr/share/squirrelmail/plugins/compatibility/index.php
/usr/share/squirrelmail/plugins/compatibility/locale/
/usr/share/squirrelmail/plugins/compatibility/locale/.htaccess
/usr/share/squirrelmail/plugins/compatibility/locale/compatibility.pot
/usr/share/squirrelmail/plugins/compatibility/locale/getpot
/usr/share/squirrelmail/plugins/compatibility/locale/index.php
/usr/share/squirrelmail/plugins/compatibility/setup.php