Historical Rules · 5 October 2018
Rule 701.23c
Official source text from the 5 October 2018 Rules release
A player may planeswalk as the result of the “planeswalking ability” (see rule 901.8), because the owner of a face-up plane card or phenomenon card leaves the game (see rule 901.10), or because a phenomenon’s triggered ability leaves the stack (see rule 704.5x). Abilities may also instruct a player to planeswalk.