Source Reference:

• Component: bbPress :: bbp-forum

  File: /bbp-includes/bbp-forum-template.php :: Trac Source Line: 1389

  			$ancestors = bbp_get_forum_ancestors( $forum_id );
  
  			foreach ( (array) $ancestors as $ancestor ) {
  				if ( bbp_is_forum_category( $ancestor, false ) && bbp_is_forum_closed( $ancestor, false ) ) {
  					$retval = true;
  				}
  			}
  		}
  
  		return apply_filters( 'bbp_is_forum_closed', (bool) $retval, $forum_id, $check_ancestors );
  	}
  
  /**
   * Is the forum public?
   *
   * @since bbPress (r2997)
   *
   * @param int $forum_id Optional. Forum id
   * @param bool $check_ancestors Check if the ancestors are public (only if
   *                               they're a category)