If Pinsir uses "Slow Crunch" and retreats or is KO'd on the opponent's next turn, does the opponent's active Pokemon still get Knocked Out?
Yes, the effect is on the Pokemon that was attacked with Slow Crunch.
Source: Twilight Masquerade FAQ; TPCi Rules Team (2024-05-23)

Relevant cards

Note: All applicable cards may not be listed below.
SV6_3
SV6_3