This isn't applied to tiered play, it's for the PSS, and thus there are no banned Pokémon. It is generally frowned upon to use cover Legendaries (Ho-Oh, Palkia, Dialga), event Legends (Keldeo, Genesect, Darkrai) or more than one "other" Legendary (Moltres, Zapdos, Suicune, Azelf), because people get it into their heads that being a Legendary automatically makes them overpowered (Meleoetta lol).