Pokémon GO trainers expected new Shadow Pokémon for the Mega Battle Event this week. None were released. This was initially thought to be a mistake, perhaps as a result of Niantic's laudable week off for company wellness, leading some trainers to expect that on Monday, when business as usual resumed, the promised Shadows would be released. However, now it seems that this confusion was a result of poor wording in Niantic's announcement.
Initially, Niantic said: "We're getting word that Team GO Rocket will be using different Shadow Pokemon during this event as well." This was interpreted to mean, by seemingly all who read it, that… well, that Team GO Rocket would be using different Shadow Pokémon during the event. It says it right there in the text, after all. However, after a week of confusion, a video from Pokémon GO YouTube content creator Mystic7, who is frequently sponsored by Niantic, cleared up the confusion.
Mystic7 said in his video:
"As we thought we knew, there was supposed to be new Shadow Pokémon, and what I thought that meant was that the Team Leaders would have new Shadow Pokémon, which would mean new Shadow Shiny Pokémon, and that there would be new Shadow Pokémon coming out of the GO Rocket Grunts. That's what I thought was happening. But that's not actually what happened. What's actually happening in Pokémon GO during the event is [Team GO Rocket is using] the same pool of Pokémon, and the ones that are weak to Bug-types are spawning at a higher frequency."
That is, the Pokémon GO community seems to all collectively agree on (a rare occurrence in itself), emphatically not what Niantic said. It seems that this was a case of poor wording, which is odd considering how specific these announcements usually are. So for those holding out hope for new Shadows, that hope is, unfortunately, at least for the Mega Battle Event, squashed.