Alex Kirilloff
It may have been because Kirilloff's debut came during a Twins' wild card game that the usual debut websites like the ones run by baseball-reference and ESPN have not picked up on him, but he did indeed make his major league debut on 9/29/2020. He played the entire game as one of the Twins' outfielders.
https://www.mlb.com/player/alex-kirilloff-666135
P.S. The only other players to ever make their major league debuts in the postseason were Mark Kiger with the A's in 2006 and Adalberto Mondesi with the Royals in 2015.