MIDI Designer Pro and Lite are ready for iOS 7 as of version 1.5.6 (released yesterday, September 17, 2013). Version 1.5.5 will work on iOS 7 and continue to look like it did on iOS 6. If you have any problems, please upgrade to MIDI Designer 1.5.6 and get in touch. [2014-06-03 Update Confirmed: Users experiences NO problems with our iOS 7 Upgrade]

But enough about MIDI Designer! What about iOS? Which devices get left behind?

Up to iOS 6 Only

iPhone 3GS
4th generation iPod touch

Up to iOS 5 Only

iPad
3rd generation iPod touch

Up to iOS 7 Only

iPhone 4 (not great with iOS 7, apparently)

Sources

Business Insider on iOS 8 Compatability
iOS 7 Compatibility
iOS 6 Compatibility
iOS 5 Compatibility