Yes, they're A/B testing multiple things with NSFW content. Up until recently I was in the "you must use the mobile app instead of mobile site to see nsfw content" group. I'm not sure exactly when that changed for me, maybe a week or so ago, but I no longer have that requirement anyway.
It's really annoying because people often use the tag in a jokey way. I see it a lot on the bread/sourdough reddits. It's funny, but the new redirect to login really breaks the assumption there.