The Pokémon Company has updated its rulebook with the Regulation Set J, which implements a number of important changes for the competitive Pokémon VGC next season. Most notably, Legendary Pokémon will finally be legal in official competition.
But with nuances. According to the rules, Pokémon from Paldea's Pokédex (1-400), Kitakami's Dex (1-200) and Blueberry's Dex (1-243) will be eligible to compete, with a few exceptions. A maximum of two Special (Mythical) Pokémon may be chosen from the list of eligible Pokémon, while another list with some old acquaintances (e.g. Mew, Mewtwo, Ho-oh, Lugia, Deoxys, Kyogre, Groudon, Jirachi, Koraidon, and Miraidon) are still restricted.
You can check out the new Regulation Set J for Pokémon VGC here. The new rules will go into effect for Ranked events in September 2025, and will be incorporated into Championships by the end of November 2025.