This has probably been theorized at some point over the past two years. But I think Nomura’s comment at the concert in 2023 may have been about Shinji Hashimoto officially retiring from Square Enix. Shinji retired the month after the anniversary event. Whatever the thing was, Nomura said it was right after that event, so the timing is right. Based on him joking with Shimomura about not knowing what he was talking about, it makes sense this is what he would have been referring to as she likely would have known about Shinji’s retirement as opposed to something crazy like Disney/SE telling them to scrap the game and restart, etc.
While it was already public knowledge at the time, the concert probably wasn’t the best time or place to get into Shinji’s retirement and all that. But it fits with the description of being something that changed the direction of the series since Shinji was literally the series’ co-creator.