I am having the same issues, the Migrator seemingly isn't being called. Single app WatchKit that I just ported from WatchKit to widgetKit, running under watchOS 10

I am migrating a WatchKit complication that I support back to WatchOS 6.0 to WidgetKit. How do I support ClockKit in WatchOS prior to 9.0 and WidgetKit in 9.0 and newer? I can't seem to get my WidgetKit Extension to compile is I have WatchOS support earlier than 9.0. The compiler complains that things like "Timeline" doesn't exist prior to WatchOS 9.0, and I don't know how to conditional that since it is a runtime decision. (can't if #available around a class definition.) I'm sure it is something simple I am missing.

An no sooner did I type the question did I find the answer. I need to set BaseSDK and supportedPlatforms to WatchOS under BuildSettings which allowed me to set Deployment Target to 9.0 under General.