These changes will be effective from January 14th 2020.
Over the last several weeks, base blue-green decks using Urza, Lord High Artificer have risen to the top of competitive Modern, earning the most 5-0 trophies in Magic Online league play and maintaining a non-mirror match win percentage of more than 55%. These decks also have a winning matchup against nine of the other ten most popular competitive decks, indicating an inability of the metagame to adjust on its own.
The cards most strongly contributing to the high win rate of these decks are Oko, Thief of Crowns and Mox Opal.
Oko, Thief of Crowns has become the most played card in competitive Modern, with an inclusion rate approaching 40% of decks in recent league play and tabletop tournaments. In additional to having a high overall power level, Oko has proven to reduce metagame diversity and diversity of game play patterns in Modern. In order to improve the health of game play and to weaken Urza decks and other top decks, Oko, Thief of Crowns is banned in Modern.
In addition to being an important part of blue-green Urza decks, Oko was also used by a number of other top Modern decks. Our data indicates that removing Oko alone would still leave Urza decks in a dominant position in the metagame. We considered options that would further weaken Urza-based artifact decks, while still allowing for decks based around that general strategy. Ultimately, we determined that banning Mox Opal was the correct option.
As a source of fast mana in the early game, Mox Opal has long contributed to strategies that seek to end the game quickly and suddenly, whether with explosive attacks, one-turn win combos, or by locking out the opponent with “prison” elements. While none of these decks previously warranted a ban of Mox Opal, it has historically been a part of decks that approached problematic impact on the metagame or did indeed necessitate other bans. As the strongest enabler in the recent Urza artifact decks, and a card that has been concerning in the past and would likely cause balance issues in the future, Mox Opal is banned in Modern.
Lastly, we'd like to take this opportunity to address another problematic interaction between Karn, the Great Creator and Mycosynth Lattice. This combination, popular in Eldrazi and other Tron decks, can completely lock the opponent out from casting further spells. While decks featuring this combination often win in other ways, the deckbuilding cost to include this interaction is low, causing it to show up more often than is fun in competitive play. As a result, we are banning Mycosynth Lattice in Modern.
For the full explanation, refer to the official announcement by WotC.