Pokémon Sun and Moon - Ony 21 (plus the Primals) of the 48 Mega Evolutions are available in SM

I think the title is a bit misleading. From what I've seen in those comments, the Megas aren't removed, just unavailable. And it's likely to just be things that will be unlocked after Pokébank. No real reasons to freak out.

Although I do find it silly how the focus on Mega was completely overshadowed by Z-Moves.