看看被接受的答案：SwiftUI Remove NavigationBar Bottom Border
之前：

之后：

import SwiftUI
struct TestView: View {
    init(){
        let appearance = UINavigationBarAppearance()
            appearance.shadowColor = .clear
            UINavigationBar.appearance().standardAppearance = appearance
            UINavigationBar.appearance().scrollEdgeAppearance = appearance
    }
    var body: some View {
        NavigationView{
            ScrollView{
                ForEach(0 ..< 20){ num in
                        Text("Num - \(num)")
                        .padding()
                }
            }
            .navigationTitle("Learn")
        }
    }
}

struct TestView_Previews: PreviewProvider {
    static var previews: some View {
        TestView()
    }
}

我在使用UIHostingController时也遇到了同样的问题，所以我最终向UIViewController添加了一个子UIHostingController，并以这种方式设置了阴影。

@IBOutlet weak var theContainer: UIView!

override func viewDidLoad() {
 super.viewDidLoad()

  let appearance = UINavigationBarAppearance()
  appearance.backgroundColor = UIColor(Color("navbackground"))
  appearance.shadowColor = .clear
  self.navigationController?.navigationBar.scrollEdgeAppearance = appearance
  self.navigationController?.navigationBar.standardAppearance = appearance

  let childView = UIHostingController(rootView: SettingsView())
  addChild(childView)

  childView.view.frame = theContainer.bounds
  theContainer.addSubview(childView.view)
  childView.didMove(toParent: self)
}