This is almost certainly being done to placate Apple, because there's no mention of Android. My guess would be that Discord is being leaned on to "clean up the NSFW problem" or be de-listed. Discord is already rated 17+ (the maximum age rating) according to https://apps.apple.com/us/app/discord-talk-chat-hang-out/id9.... I don't see how rating it 12+ would ever be possible given the description at https://apps.apple.com/sa/story/id1440847896?l=en.