At the moment, Capacitor supports iOS 11+.
Apps just won’t install on older versions. Xcode won’t allow you to run apps in older devices, and if distributed on the App Store it won’t appear to download in older devices.
Latest cordova (5.x.x) supports iOS 10+, but next major release (6.x.x) is supposed to only support iOS 11+.
So if you need to support iOS 9+, you’ll have to use cordova 4.x.x or build your own thing.