Over the past several weeks, the Standard metagame has been in an unhealthy state, so we're taking significant steps to rectify it. The two major issues are the dominance of Simic-based Food decks featuring Oko, Thief of Crowns and the general overrepresentation of green in the competitive metagame.
Food decks featuring Oko, Thief of Crowns have been the most popular and most winning for the majority of the Throne of Eldraine Standard season. This culminated with almost 70% of decks at Mythic Championship Richmond including the card. Based on data from high-ranked Arena traditional (Best-of-Three) play, only one of the other ten most-played decks (Simic Flash) had a favorable matchup against Simic-based Food decks, and only just above 50%. Food decks maintained an average of about a 53% non-mirror match win rate, even with the metagame focused on beating them.
Oko, Thief of Crowns has also reduced metagame diversity and diversity of gameplay in Standard by shutting off build-around creatures and artifacts. Ultimately, Oko's power level has proven higher than is healthy for the current metagame, and higher than intended for future environments, including Theros: Beyond Death and forward.
To address green's general dominance, we're also choosing to remove Once Upon a Time and Veil of Summer from the environment. Alongside Oko, Thief of Crowns, Once Upon a Time is one of the key reasons green has been overrepresented in the environment. It contributes to a high consistency of strong starts and provides a level of early mana fixing that other colors don't have access to. This advantage is especially important in the context of a small five-set Standard card pool with less flexible mana bases. Arena data indicates that, without also removing Once Upon a Time, green decks would still continue to be too powerful and consistent going forward.
Finally, Veil of Summer is also playing an important role in preventing the metagame from being able to self-correct. Cards that played similar roles in the past, like Autumn's Veil and Display of Dominance, proved a lower power level than desired in their respective Standard environments, leaving green with a weaker option compared to the other "color hate" cards in those cycles. Veil of Summer is at the other end of the spectrum. It's too much more efficient than the other cards in its cycle, and by comparison to other tools available in Standard, gives green decks too much resilience against removal and disruption.