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Rest assured, theyre not gone

It's intended to help expose a player to new playstyles or inspire them to build new Decks

13 of the JP-alt art sleeves were released for War of the Spark, so those images can be viewed there (April 2019)

Borrowing the East Wind is quite the answer

If you cast a spell for which mana may be spent as though it were mana of any color, you may cast it for Leyline of Mutation's second ability's alternative cost and still spend mana as though it were mana of any color

If you just want that ability, Basri's Solidarity costs a lot less mana, and you can find more repeatable effects like some Ajani planeswalkers