Neither Mega Evolutions nor Z-Moves will return in Pokémon Sword and Shield, developer Game Freak confirmed during a Nintendo Treehouse: Live presentation at E3.
Mega Evolutions, introduced in Pokémon Omega Ruby and Alpha Sapphire, allowed Pokémon to transform into, ostensibly, their ultimate forms, and the evolutions even changed their types, abilities and stats. Z-moves were introduced later in Pokémon Ultra Sun and Ultra Moon, and let players unleash mighty, devastating attacks during battles.
These abilities might be gone, but developer Game Freak is introducing a new feature called Dynamax in their stead. Certain locations in Sword and Shield's Galar region allow Pokémon to Dynamax, which makes them massive in size for three turns and increases the power of their abilities.
So, rest in peace, Mega Evolutions. And hello, parade float Pokémon!