Thank you, BTW, do you know how to find the corresponding git commit / branch given the version number like this "swiftlang-5.9.0.114.6"?
I didn't encounter any major blockers and managed to compile Observation sources for older OS relatively easy with some unimportant changes having to do with compilation directly from my app. When the app runs I can see that accesses properly recorded on getting state bits and withMutation keypaths properly recorded when state is changed. I can not test it fully though, as SwiftUI on that older OS release doesn't know it now needs to call "ObservationTracking._AccessList._installTracking" (perhaps among other things).
Hopefully SwiftUI with the new Observation can be relatively easily back deployed to the previous OS releases, it's just up to Apple as SwiftUI is not open source.