It’s been a huge effort to rewrite the system in place as seamlessly as possible. One of our first steps was building landscape support, which we shipped for the iPhone 6 Plus earlier this summer. From this effort, we learned a lot about how and when to adapt to changing conditions. This was the foundation on which we built our updated iPad support. Now, we have new tools that are more robust and allow faster iteration. Adaptive UI gives us the ability to support new devices and bring new Twitter features to people faster. But as you might expect, from time to time there are going to be some differences in features and functionality between devices. In the future, we aim to get Twitter for iPad and iPhone to complete feature parity.
Twitter says there's no longer Twitter for iPhone and Twitter for iPad, rather there's Twitter for iOS and it "will now be optimized for different contexts "
More details on the new design can be found here. You can download Twitter from the App Store for free.