It seems it has to do with the iOS takedown that Tumblr received.

As 9to5 Mac noted, the issue may have something to do with inappropriate content that violated Apple’s guidelines, which has led Apple to pull apps before. The Verge pointed to a Twitter thread that appeared to indicate inappropriate content was at the heart of the issue.

Per Apple’s App Store Review Guidelines, objectionable content includes content that’s “offensive, insensitive, upsetting, intended to disgust, or in exceptionally poor taste.” It’s worth noting that “objectionable content,” however, could be interpreted to mean any number of things.

source

There's also lots of people and artists complaining at Twitter if you search for 'Tumblr NSFW'

Grab your favorite artists' posts and links to other sites while you still can.