Historical Rules · 25 January 2019
Rule 508.5
Official source text from the 25 January 2019 Rules release
If an ability of an attacking creature refers to a defending player, or a spell or ability refers to both an attacking creature and a defending player, then unless otherwise specified, the defending player it's referring to is the player that creature is attacking, or the controller of the planeswalker that creature is attacking. If that creature is no longer attacking, the defending player it's referring to is the player that creature was attacking before it was removed from combat or the controller of the planeswalker that creature was attacking before it was removed from combat.