Additionally, the prevention of certain legendaries from battle facilities has always been the rule since the Battle Tower was first introduced in Crystal.
During the online Reborn League, after challengers had beaten it there was a special challenge afterwards they could do called the X-League where both the challenger and leaders could use a legendary on their team, including Ubers. I wonder if the X-League will be implemented in the postgame. Ame has confirmed there will be Gym Leader rematches before, saying it wouldn't be much of a postgame if there weren't.