SwiftUI is not compatible with earlier versions of iOS, macOS, or any of Apple’s other operating systems. So, to start adopting it, you either need to decide that it’s acceptable for the app in which you’ll use SwiftUI to only work on the latest operating systems — or you’ll need to ”gate off” the features written in Swift UI from older versions, using Swift’s @available syntax.
I found this post by @johnsundell to be a very useful, concise explanation of what I need to know right now about Swift UI.
Right off the bat, it tells me that I can wait a little while to adopt it. My time this year would be better served refactoring my codebase to prepare for a SwiftUI transition next year. For example, I can already start work on getting rid of storyboards in SwiftoDo for iOS, and that will prepare me for the storyboard-free future with SwiftUI.