Hm.. to facilitate your requirement you can add jQuery .on('click') listener to every link of a topic from NSFW category.
It would trigger a custom popup requiring confirmation before going further.
You could even store confirmation in a cookie, so that on this specific machine user does not have to confirm each time.
I don’t know if such feature will arrive to upstream any time soon.
If you really need this feature custom JS is the easiest way to go IMO.
If you need more specific help (eg. custom code for your forum) feel free to PM me 