Question: Which Magic: The Gathering Standard card (non-foil paper version released in its original set) that was banned at the same time as Oko, Thief of Crowns (including Oko, Crown of Thorns) had the highest price decrease from its all-time high to its all-time low?
Answer: Oko, Thief of Crowns
Explanation: First, you need to find the cards that were banned when Oko, Thief of Crowns was banned in the standard format. Wizards.com has archives for card bannings. The website shows that three cards were banned in the Standard format on November 18, 2019: Oko, Thief of Crowns,
Once Upon a Time, and Veil of Summer. A website like Mtggoldfish has historic prices for cards. Enter the card name in the search box (make sure to click on the non-foil version and the original set, for instance, a quick Google search shows that Oko, Thief of Crowns, was first released in Throne of Eldraine. Navigate down the page, and you will see a tab that says tabletop or MTGO. Hit tabletop and look at the chart, which shows all-time high and all-time low. Oko has 67.61 and 11.90 respectively. The numbers for Once Upon a Time are 19.31 and 0.75 respectively, Veil of Summer had the numbers 13.30 and .30 respectively.