Im pretty sure they mean planes walkers, but what are Magic players if not pedantic
At the end of the day, is there really anything worse than watching your opponent play your own cards against you
Its obviously a much less powerful card overall, and you cant just throw it into a deck with no concern for what else is in there, but its possible there is a deck built with The Cauldron of Eternity in mind and then this deck could want Gray Merchant, but it really has to be a dedicated self-mill deck for it to work, because if you cast Gray Merchant and it dies naturally (which is the most common play pattern), it doesnt work with the Cauldron unless the Cauldron isnt on the battlefield yet
1999s Masticore took this concept to the extreme, forcing regularly discards in exchange for a creature that could kill enough things to arguably recoup the loss
Blue is my favorite color to play in Magic, and even though creatures arent usually at the forefront of blue decks, it still has some of the most powerful creatures in the game
Considering that some of these cards are Legacy and Vintage all-stars, it only seemed right that a ban would follow up on their announcements