If I use Sylveon's "Wink Wink" attack and discard my opponent's Diantha, can I get 2 cards from my discard pile even it 1 of my {Y} Pokemon was not Knocked Out on my Opponent's last turn?
No, you still have to meet the requirements of the card's text. You can discard the Diantha, though.
Source: Forbidden Light FAQ (2018-04-20), TPCi Rules Team (2018-05-03)

Relevant cards

Note: All applicable cards may not be listed below.
SM6_87