Resolves YES when the phrase, "you may cast this card without paying its mana cost," or similar is packed into a keyword in the Comprehensive Rules AND a card uses the keyword.
The keyword must not be conditional on prior conditions being met. For example, a keyword like Plot that says you can cast the spell without paying its mana cost if you paid the Plot cost on a previous turn would not count. It must only be trying to convey that the spell can cast for free, no strings attached.
Resolves NO if no card meets this criteria by 11:59pm EST Dec 31, 2029.
Fine print:
Resolves to the year where the Comprehensive Rules contains a keyword that meets this criteria AND a card has Oracle text that contains it.
Errata'ing past cards counts as soon as a Scryfall page shows the wording.
The keyword must be available on physical cards, not just digital only cards.
Past cards do not need to be errata'd for this market to resolve as long as a new card contains the keyword and the Comprehensive Rules explain the keyword.
Silver bordered cards, acorned cards, and Mystery Booster playtest cards do not count.
Additional context:
Mill was keyworded in 2020 with Core Set M21.
In Oct 2022, cards that did all of the actions of Surveil without being technically Surveil were errata'd to Surveil, such as Consider and Otherworldly Gaze.
It appears that "enters that battlefield" will be retemplated to "enters" in Duskmourn according to spoilers. (edit: It happened in Bloomburrow.)
@traders I have added a stipulation that Mystery Booster playtest cards do not count.
I hope this is not a controversial addition since I have disqualified silver border and acorned cards already.