@timbre's answer makes many good points and is worth an up-vote. But it didn't answer exactly what I needed. So here's what I came up with.

See this article for how I came up with 6.0 corresponding to target SDK macOS 15

#if swift(>=5.0)
    ...swift code...
#else
    ...old approach...
#endif


#if canImport(SwiftUI, _version: "6.0") //hack to test for macOS 15 target sdk
    ...swift code...
    ...perhaps using #available if deployment sdk is older...
#else
    ...old approach...
#endif