We all knew it was inevitable, but it's now official: the card Oko, Thief of Crowns has been banned from Pioneer, after nearly tearing the Magic scene apart at the seams. According to Ian Duke of WotC, Simic Food Ramp decks have simply become too powerful, with a 60% win rate in Magic Online and a prominent 5-0 winner in leagues; additionally, Oko appeared in a number of other powerful decks, leading to his ban.
In addition to the Simic Food Ramp deck, the Simic Nexus deck has also proven itself to be outsized in terms of strength, with the second-highest win rate and a number of 5-0 finishes as well. To counter this, WotC is banning the card Nexus of Fate, which let players take an additional turn and then shuffle the card back into their deck.
Both bans will go into effect on December 17, and the next announcement will occur on January 6, 2020. The full list of cards banned from Pioneer can be found at this link.