Not just porn! Since reddit only has one such tag you can apply to posts, many subreddits were using the NSFW tag for a wider variety of reasons such as spoilers, content which may contain unsettling material (aka "triggers"), and more!
The stoners on Reddit got screwed - many of the weed subs got the banhammer, and the rest got NSFW'd because spez wants Reddit as G-rated as Disney for the IPO.
To be fair, there's been a site wide spoiler tag for a few years now, but yeah before it was introduced people used NSFW for that if a community didn't allow actual NSFW stuff or whatever
It’s not all NSFW. The Synology community I followed went full NSFW in protest and I can still view their subreddit. I cannot view subreddits such as watchitfortheplot. So somehow Reddit is differentiating.
Well I know they forced some subreddits go back to SFW if it was obvious the NSFW was just to protest. Maybe that's the case for the subreddit you're looking at?