Well, there were a bunch of audio changes with iOS 13, including this one which sounds like it could be important for Keystage, now that it acts as an AUv3 host:
"Audio Unit Extensions now support user presets that are available across all host applications."
A bunch of other audio stuff too... check: https://developer.apple.com/documentation/ios-ipados-release-notes/ios-13-release-notes
iOS13 requirement comes from the AVAudioEngine framework of Apple that I used in KeyStage 2.0 for AUv3 hosting and audio mixing.