Square Enix removed The World Ends With You: Solo Remix from iTunes after a bug made it incompatible with iOS 8. The publisher said they would fix the issue, but it would take some time since it was more complicated than expected.
It appears Square Enix fixed the issues since The World Ends With You: Solo Remix is available again in Japan. A note says that version 1.21, the current release, is compatible with iOS 8.
The World Ends With You: Solo Remix is a revised version of the Nintendo DS game with the same story, but without dual screen battles. The graphics have been enhanced and more songs are in the mobile version.