In a post on its website, Tumblr does not state the reason as to why the app might have been removed from the App Store, but have stated that they are working to restore it. According to the post, “We’re working to resolve an issue with the iOS app and hope to be fully functional again soon. We really appreciate your patience as we figure this out, and we’ll update this article when we have news to share.”
The latest update which was made on the 17th of November continues to mention that they are still working on the “issue” with the app, but once again they do not state the reason why. That being said, this would not be the first time that an app has been removed for hosting inappropriate content.
For example earlier this year the messaging app Telegram temporarily disappeared from the iOS App Store. It was later discovered that it was because some users were using the app to spread child pornography, but once Telegram dealt with the issue, the app was restored.