Historical Rules · 8 June 2018
Rule 719.5a
Official source text from the 8 June 2018 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.