2019-11-12 : If a Mythic Championship is in progress during your game, Metagamer’s effect affects spells that share a name with the winning deck in the previous Mythic Championship. If the Mythic Championship concludes while your game is in progress, the spells affected by Metagamer’s effect will immediately change.

2019-11-12 : Metagamer’s effect affects spells that share a name with cards in the winning deck, not just the physical cards played in that deck.

2019-11-12 : Normally there is only one winning deck for each Mythic Championship. If that event was a team event, there may be more than one.

2019-11-12 : The last Mythic Championship will be (or was) Mythic Championship VI in November 2019. After that event, Metagamer will receive errata so that it refers to the winning deck from the latest Players Tour, Players Tour Final, or Mythic Invitational. If a new tournament series comparable to those is announced in the future, the rules text will be updated again. This errata won’t be reflected in the displayed card text.

2019-11-12 : The “winning deck” is the deck that was played by the winner of the finals. That is, if the event uses multiple formats (such as a Constructed format and a Booster Draft), whatever format was played in the finals determines what the “winning deck” is.

2019-11-12 : You can find more information on the latest Mythic Championship, including decklists, at MTGEsports.com.