What version of Swift is SwiftUI written on?

Using Swift
3

SwiftUI was introduced in iOS 13 which bundled a toolchain with Swift 5.1

SwiftUI has progressively been updated to the latest versions like as of recently SwiftUI adopted Swift 6 concurrency.
SwiftUI is updated with each new iOS release to take advantage of features and optimizations meaning that it has been developed on code from Swift 5.1 all the way to 6.0

TLDR
SwiftUI is a System library for Darwin based OSes. These libraries are updated every time there is an update to be maximally compatible with the latest system provided runtime version. SwiftUI has been available since iOS 13 which bundled Swift 5.1

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