Mox Opal

• WWizards of the Coast2020-08-07
  Official Ruling
  If you control Mox Opal and one other artifact, then have a second Mox Opal enter the battlefield as your third artifact, you can't activate either one's mana ability before putting one into your graveyard due to the “legend rule.”

Rulings via Scryfall

• WO
  Wizards of the Coast2020-01-13

  Mox Opal was banned in modern on January 13, 2020. 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. 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. Source: https://magic.wizards.com/en/news/news/january-13-2020-banned-and-restricted-announcement
• WO
  Wizards of the Coast2024-12-16

  Mox Opal was unbanned in modern on December 16, 2024. Our expectations are that the reintroduction of Mox Opal will give rise to several interesting artifact strategies both new and old without adding any power to strong decks. And so, Mox Opal is unbanned in Modern. Source: https://magic.wizards.com/en/news/announcements/banned-and-restricted-december-16-2024