Fair question. This still needs to be properly implemented and thought through, but the likely direction is:
When posting, users will be able to mark content as NSFW. Other users will then be able to choose in their profile settings whether they want to see NSFW content or hide it.
On top of that, moderators of a Substr will be able to mark posts as NSFW as well. I also want some community input there, so if enough users flag a post as NSFW, it can receive that status too.
So yes, likely allowed in some form, but with user controls and moderation around it.