That does the trick alright! Thanks a bunch.
For our purposes we needed to apply it to more than one forum. Just applying it to a parent category (didn't try a parent forum though) won't recursively apply it to the forums underneath it, so we applied every forum id manually with this slightly modified snippet:
$forumIDs = array(17, 42, 999);
Oh, and for other tech-illiterates like myself, you're gonna want to put this code in your child-theme's functions.php file (do share if there are better alternatives, but this worked for me) and you'll find a forum's ID by going to
/wp-admin/edit.php?post_type=forum and mousing over or opening a forum to reveal it's "post" number, i.e. it's id, for example
post=197975. And yes, we had a number that large, almost definitely as a result of our conversion from the BuddyPress legacy forum.