From: https://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=45551&start=10#p264315

The canonical URLs of topics don't have the f variable which means that there are users who are viewing the topics in their canonical form (without the f=) such as those who found them on Google. Those users are not displayed in the who is online list of the specific forum.

Other than that, there's no other drawback in removing the f as it's totally useless.