As part of their gradual Pioneer banlist rollout, Wizards of the Coast has announced that three more cards will be banned in the new format. These cards are Once Upon a Time, Smuggler's Copter, and Field of the Dead.
According to WotC, the Once Upon a Time card is a major contributor to the heavy prevalence of green decks. Similarly, Smuggler's Copter is making aggressive decks, and especially mono-black decks, too powerful for their liking. Lastly, Field of the Dead is suppressing controlling and reactive decks, and WotC would like to see more of those in play.
Writing for WotC, Ian Duke stated that the slow rollout of the Pioneer banned and restricted list has been a "positive experiment" so far, and has led to a smaller and more accurate list of banned cards. Updates will be slowing down as the year ends, and eventually the Pioneer updates will be synced with the updates to all formats.
The next B&R announcement is scheduled for December 9. The full list of cards banned and restricted in Pioneer can be found here.