Retire Legacy Forums.
"Legacy Forums" is what we now call the bundled version of bbPress 1 that has shipped with BuddyPress for over nine years. The Legacy Forums codebase/features are many years stagnant, and it has been almost completely hidden from the UI for the past five years.
Legacy Forums were replaced by bbPress 2, which is its own plugin, and we've been promoting its integration with BuddyPress for a long time. Most significantly, bbPress 1 only runs on WordPress versions older than 4.7, because of a BackPress conflict (which is nested inside bbPress 1) with WordPress 4.7's WP_Taxonomy
class.
If your site is still using Legacy Forums, you will need to migrate to bbPress 2 to run BuddyPress 3.0. See https://bpdevel.wordpress.com/2017/12/07/legacy-forums-support-will-be/ for more information.
Fixes #5351
See #7502