Are Xcode library Custom views/modifiers iOS14 only?

The API that is used to declare the items (LibraryContentProvider etc) is iOS 14, but you can guard against it's usage when building for iOS 13 with @available and Xcode will still pick it up correctly, so the following compiles for iOS 13 and still populates your library.

Code Block 
@available(iOS 14.0, *)
struct LibraryContent : LibraryContentProvider {
  @LibraryContentBuilder
  var views: [LibraryItem] {
    LibraryItem(
      Text("Hello WWDC!"),
      title: "Custom View In The Library"
    )
  }
}