Historical Rules · 5 October 2018
Rule 901.14b
Official source text from the 5 October 2018 Rules release
If a player would leave the game and that player leaving the game would reduce the number of turn markers in the game, that player first ceases to be a planar controller (but no other player becomes a planar controller), then that player leaves the game. Each face-up plane card or phenomenon card that player controlled is put on the bottom of its owner’s planar deck. No player is considered to have planeswalked.