From the most recent patch notes:
Casting Sphinx’s Decree no longer causes a soft-lock to occur, if the opponent has at least one Spell card in their hand.
Casting Sphinx’s Decree still soft-locks.
I’m playing as Karn vs. Chandra, Roaring Flame. She has 3 cards in hand. I cannot tell if they are spells, obviously, but I play Sphinx’s Decree and the game soft-locks immediately. I have Brawler’s Plate, Traxos, Scourge of Kroog (triggered this turn by playing Brawler’s Plate), and Detection Tower in play, and I played Sphinx’s Decree off of a bonus match cascade. Not sure if any of that information is helpful, but that’s all the info that I have on what was happening in the game at the time.
I really want to play this card in my Karn deck, because it completes a control Exodia setup that I have that disables all creatures and prevents spells from being cast, but I guess I’ll keep waiting for this card to actually be fixed.
EDIT:
Client Version: 2.9.0.26959
Data Version: 2.9.0.26971
Server Version: 2.6.0
//Edited Title -Brigby