I Installed the iOS 9.3 simulator in Xcode 10.2; available iDevices show up as both iOS 12.2 and 9.3 targets, and I was able to successfully build an app for iOS 9.3.
When I invoke my project in Xojo 2019 r1.1, with the developer tools targeted at Xcode 10.2 (i.e. using xcode-select), the iOS 9.3 simulator targets do not appear as targets under iOS Debugging>Simulator Devices - just the 12.2 targets.
I must target baseline iOS 9.3 devices as many of our customers use older devices (iPad Retina, for example), yet neither the Apple Store nor TestFlight accept apps for distribution other than those built with the Xcode 10.2 toolset.
So, according to Xcode 10.2, iOS 9.3 simulator devices are available, but not available under Xojo. The iOS 9.3 devices *are* also available in the standalone simulator app. What's missing?