Conversation about back deployment has been practically rendered off-topic discussion:
This reason was given for why Observation cannot be back deployed:
As far as I can tell, no technical reason has been revealed. Also, it seems like there could be at least one way it could be made to work with older SwiftUI runtimes. That said, I trust the engineers at Apple have thought of things like this and considered them seriously. They did go to a lot of effort to back deploy Swift Concurrency, after all.
We'll just have to wait for our respective user bases to hit a certain level of adoption and hope the tech debt is not prohibitive.