/usr/share/php/sabre21/Sabre/CalDAV/Notifications/ is in php-sabre-dav-2.1 2.1.10-1ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/share/php/sabre21/Sabre/CalDAV/Notifications/Collection.php
/usr/share/php/sabre21/Sabre/CalDAV/Notifications/ICollection.php
/usr/share/php/sabre21/Sabre/CalDAV/Notifications/INode.php
/usr/share/php/sabre21/Sabre/CalDAV/Notifications/INotificationType.php
/usr/share/php/sabre21/Sabre/CalDAV/Notifications/Node.php
/usr/share/php/sabre21/Sabre/CalDAV/Notifications/Notification/
/usr/share/php/sabre21/Sabre/CalDAV/Notifications/Notification/Invite.php
/usr/share/php/sabre21/Sabre/CalDAV/Notifications/Notification/InviteReply.php
/usr/share/php/sabre21/Sabre/CalDAV/Notifications/Notification/SystemStatus.php
/usr/share/php/sabre21/Sabre/CalDAV/Notifications/Plugin.php