When I use Galarian Slowking V's "Word of Ruin" attack, is my opponent's Pokemon still Knocked Out if they retreat it during their next turn?
Since it is an attack effect on the Defending Pokemon, the effect is removed if that Defending Pokemon is no longer Active.
Source: Chilling Reign FAQ; TPCi Rules Team (2021-06-17)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH6_99
SWSH6_99