Is SwiftUI backwards-compatible with iOS 12.x and older?

Question

If I have an app made with SwiftUI, will it work for iOS below iOS 13?

Answer 1

I just checked it out in Xcode 11 and can confirm it won't be backwards-compatible, as can be seen in SwiftUI's View implementation:

/// A piece of user interface.
///
/// You create custom views by declaring types that conform to the `View`
/// protocol. Implement the required `body` property to provide the content
/// and behavior for your custom view.
@available(iOS 13.0, OSX 10.15, tvOS 13.0, watchOS 6.0, *)
public protocol View : _View {

    /// The type of view representing the body of this view.
    ///
    /// When you create a custom view, Swift infers this type from your
    /// implementation of the required `body` property.
    associatedtype Body : View

    /// Declares the content and behavior of this view.
    var body: Self.Body { get }
}