https://www.gamespot.com/articles/competitive-pokemon-explained-ultra-sun-moon-editi/1100-6452650/

Additionally, many legendary and mythical Pokemon are prohibited. A big change from previous years is that Kyogre, Groudon, and Rayquaza are banned--a welcome change for those who were tired of their weather effects and the meta-game that sprung up around them. Mega Evolution is also back after last year's omission.