Historical Rules · 22 April 2021
Rule 720.5a
Official source text from the 22 April 2021 Rules release
At the end of a subgame of a Planechase game, each face-up plane card or phenomenon card is turned face down and put on the bottom of its owner’s planar deck. Then each player moves their planar deck from the subgame command zone to the main-game command zone and shuffles it.