Tumblr has also confirmed that this was the issue in a statement that reads, “We’re committed to helping build a safe online environment for all users, and we have a zero tolerance policy when it comes to media featuring child sexual exploitation and abuse.” They also offered an explanation as to why some content slipped past its built-in filters.
“A routine audit discovered content on our platform that had not yet been included in the industry database. We immediately removed this content. Content safeguards are a challenging aspect of operating scaled platforms. We’re continuously assessing further steps we can take to improve and there is no higher priority for our team.” Note that this was also the same reason as to why Telegram was previously removed from the App Store (it has since been reinstated).
As it stands the app has yet to be returned to the App Store, so if you want to access Tumblr on your iOS device, you’ll have to do so using a mobile browser like Safari.