/usr/share/squirrelmail/plugins/ is in squirrelmail-logger 2.3.1-1.
This file is owned by root:root, with mode 0o755.
..
/usr/share/squirrelmail/plugins/squirrel_logger/
/usr/share/squirrelmail/plugins/squirrel_logger/alert_functions.php
/usr/share/squirrelmail/plugins/squirrel_logger/config.php
/usr/share/squirrelmail/plugins/squirrel_logger/database_functions.php
/usr/share/squirrelmail/plugins/squirrel_logger/docs/
/usr/share/squirrelmail/plugins/squirrel_logger/docs/.htaccess
/usr/share/squirrelmail/plugins/squirrel_logger/docs/index.php
/usr/share/squirrelmail/plugins/squirrel_logger/functions.php
/usr/share/squirrelmail/plugins/squirrel_logger/index.php
/usr/share/squirrelmail/plugins/squirrel_logger/locale/
/usr/share/squirrelmail/plugins/squirrel_logger/locale/.htaccess
/usr/share/squirrelmail/plugins/squirrel_logger/locale/index.php
/usr/share/squirrelmail/plugins/squirrel_logger/logger
/usr/share/squirrelmail/plugins/squirrel_logger/setup.php
/usr/share/squirrelmail/plugins/squirrel_logger/show_stats.php