@since: bbPress (r3291)
Description
bbp_exclude_forum_ids()

Returns a meta_query that either includes or excludes hidden forum IDs from a query.

Source: bp-forums/forums/functions.php:1834

bbp_pre_get_posts_normalize_forum_visibility()

Adjusts forum, topic, and reply queries to exclude items that might be contained inside hidden or private forums that the user does not have the capability to view.

Source: bp-forums/forums/functions.php:1934