I’m not seeing the option either right now; I just tried it. Since I’ve uploaded NSFW content in the past, I know it works with my account. I suspect it’s either a bug or they’re tweaking the code. They’re currently implementing spam protection on MakerWorld and here on the forum. I think there’s still some work to be done.
Maybe the flag will be converted into a tag in the future, but that’s just a theory. The whole system has never really worked properly anyway. I’ve already received warnings telling me to label my NSFW content, even though I’ve been doing that for months. So false alarm.
I even started a thread, but for some reason MakerWorld isn’t showing any interest to fix this… or they fixed it at some point, but never mentioned it. The whole NSFW flag is completely broken - it always has been.