ma There was no particular issue, it was just intended to support iPhone 5s or newer devices, and iPhone 5s supports up to iOS 12. Stability and reliability were the most important requirements, and it felt like sticking to newer iOS versions would help in that matter.
You will also note that it elegantly handles the multiple keyboard types on iPad (Floating, Undocked, Split).
Feel free to fork it and experiment with other iOS versions 😀