While Pokemon Sword and Shield will include Dynamaxing, an ability that allows a Pokemon to become gigantic and powered up during a fight, it won’t be bringing two other battle mechanics back. Junichi Masuda, the producer of these two installments, said that Mega Evolutions and Z-Moves will not be returning. This was also confirmed during Nintendo Treehouse: Live at E3 2019. (Thanks, Famitsu!)
Mega Evolution was introduced in Pokemon X and Y. If a person has a keystone for a Pokemon, such as Blastoisinite, that creature can Mega Evolve into a new version of themselves with an altered appearance, different ability, and sometimes even different types.
Z-Moves were a mechanic found in Pokemon Sun, Moon, Ultra Sun, and Ultra Moon. These are special moves for certain Pokemon that could only be used once per battle.
Pokemon Sword and Shield will come to the Nintendo Switch on November 15, 2019.