"Special Pokemon", Mythical Pokemon, and Ash-Greninja are banned by default unless specifically unbanned. Past that, there are a few other cases.
2008: Dragonite and Tyranitar
2009: All forms of Rotom
2011: Pokemon not in the Unovan Pokedex
2013: Chatot
2014: Pokemon not in the Kalosian Pokedex
2017: Pokemon not in the Alolan Pokedex (including Kantonian forms of Alolan Pokemon)
2019 (Series 1): All Gigantamax Pokemon
2020 (Series 2): Gigantamax Coalossal, Lapras, Flapple, Appletun, Alcremie, Kingler, Orbeetle, Grimmsnarl, Hatterene, Solar Power Charizard, Toxtricity, Machamp, Gengar, Garbodor, Copperajah, and Duraludon
2020 (Series 3): Gigantamax Machamp, Gengar, Garbodor, Copperajah, and Duraludon
2020 (Series 6): Venusaur, Gyarados, Porygon2, Tyranitar, Torkoal, Hippowdon, Magnezone, Togekiss, Excadrill, Whimsicott, Incineroar, Mimikyu, Rillaboom, Cinderace, Indeedee, and Dragapult
2022 (Regulation A): Pokemon not in the Paldean Pokedex, Paradox Pokemon, and the Treasures of Ruin
2023 (Regulation B): Pokemon not in the Paldean Pokedex, the Treasures of Ruin
2023 (Regulation C): Pokemon not in the Paldean Pokedex
2023 (Regulation D): Pokemon in the Kitikami Pokedex, Walking Wake, and Iron Leaves
2023 (Regulation E): Pokemon in the Blueberry Pokedex, Walking Wake, and Iron Leaves
2024 (Regulation H): Legendary and Paradox Pokemon
World Championships | Video Game Championship (Bulbapedia)