It looks like Tumblr breached their contract with Apple, possibly due to it’s content. I heard there were issues with the filtering not preventing explicit content from being being displayed to minors and anyone not wanting to view said content.
There are no issues if you have the app already installed on your Apple device. You will only have an issue if you delete the app, and try to add it back from the Search view.
You can still download the app by opening the App Store,
Click on Updates, Click on your avatar, click ‘Purchased’ link, then search for Tumblr. Now you can add the app back to your iOS device.
Sharp Dressed Dom 