The World Ends With You: Solo Remix has been updated with iOS 9 support. Better late than never, I guess?
This follows the debacle surrounding the iOS 8 update, when the game stopped working, and Square said it wouldn't be updating it.
The game was pulled from sale, Square retracted its statement, and then eventually put an updated, working version back on the App Store.
Anyway, if you were until the game was updated to upgrade your OS, your wait is over. Go ahead, get iOS 9 and its slightly weird fonts, we won't judge you.