In phpbb/notification/method folder there is the Board.php file.

There is a public function in it called get_notified_users that reads the Notifications table.

If the site admin chooses to not have Notifications removed from their forum, then the Notifications table will constantly increase. 

The SQL in this public function appears to be inefficient as the Notifications table doesn't include the column 'item_parent_id' as an index. Its possible that other columns may need to be indexed.