From; https://www.serebii.net/index2.shtml

"The first images from CoroCoro have leaked and have revealed the latest news. As always, do note that translations are preliminary while we work on them. In this, it confirms that Mega Evolutions return in the games once again activating when you have the Key Stone and the Pokémon's associated Mega Stone. The magazine also showcases Vermilion City Gym and showcases that Lt. Surge once again has a Raichu . We'll bring more as it come."

What do you mean by "return"? They never went away. This might as well be Sun & Moon-like post-game mega evolutions.

They don't have to leave to return. This isn't a typical game and it only features the original 151 Pokemon. A lot of people were speculating mega evolution might not return. (People were also thinking held items wouldn't return as well for some reason).

Omg, why isn't this spoiler tagged!!!!

xD

This is really good news. I was not expecting them to return, and this gives more credit to them adding brand new megas.

I think some are pretty good, makes a few Pokemon relevant again like Pidgeot and Beedrill.

Mega beedrill design is Ace. So good. I get some mega (ampharos) look worse than base form but many look better 

Charizard X

Banette

Abamanosnow are examples

FTFY

So you want Z moves instead?

Z moves were awful compared to Mega Evolution! Happy to see its return!