Historical Rules · 11 November 2016
Rule 718.5a
Official source text from the 11 November 2016 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 his or her planar deck from the subgame command zone to the main-game command zone and shuffles it.