Historical Rules · 17 April 2020
Rule 508.4b
Official source text from the 17 April 2020 Rules release
If the effect that states a creature is attacking specifies it’s attacking a certain player, and that player is no longer in the game when the effect resolves, the creature doesn’t become an attacking creature. The same is true if the effect specifies a creature is attacking a planeswalker and that planeswalker is no longer on the battlefield or is no longer a planeswalker when the effect resolves.