According to the note that Tumblr attached to the app, it reads, “This particular update also includes changes to Tumblr’s Community Guidelines which prohibit certain kinds of content from being shown on Tumblr.” Neither Apple nor Tumblr have officially commented on the return of the app, but the note seems to be pretty telling.
Tumblr had previously announced that they would be banning all adult content from its platform. It was a controversial move and while Tumblr does play home to reuploaded bits of porn, there are also many artists who have relied on the platform as being a safe space of sorts where they can upload their work without worrying about it being taken down or censored (assuming it’s legal, of course).
There have since been multiple reports suggesting that Tumblr has made a mistake with their latest decision, although whether or not that is true in the long run remains to be seen.