See https://twitter.com/simonbs/status/1511406796078030851 for context.

Example.swift

import SwiftUI


struct SettingsView: View {
    
    @State var isPortraitOnlyViewPresented = false
    
    var body: some View {
        List {
            Section {
                Button {
                    isPortraitOnlyViewPresented = true
                } label: {
                    Text("Present Portrait-Only View")
                }
            }
        }
        .background(PresenterView(isPortraitOnlyViewPresented: $isPortraitOnlyViewPresented))
    }
}


struct PresenterView: UIViewControllerRepresentable {
    
    @Binding var isPortraitOnlyViewPresented: Bool
    
    func makeUIViewController(context: Context) -> PresenterViewController {
        return PresenterViewController()
    }
    
    func updateUIViewController(_ uiViewController: PresenterViewController, context: Context) {
        if isPortraitOnlyViewPresented {
            uiViewController.present()
        }
    }
}


class PresenterViewController: UIViewController {
    
    func present() {
        let viewController = ExamplePortraitOnlyViewController()
        viewController.modalPresentationStyle = .fullScreen
        present(viewController, animated: true)
    }
}


class ExamplePortraitOnlyViewController: UIViewController {
    
    override var supportedInterfaceOrientations: UIInterfaceOrientationMask {
        .portrait
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .systemBackground
        let action = UIAction { _ in self.dismiss(animated: true) }
        let button = UIButton(type: .system, primaryAction: action)
        button.setTitle("Dismiss", for: .normal)
        view.addSubview(button)
        button.translatesAutoresizingMaskIntoConstraints = false
        button.leftAnchor.constraint(equalTo: view.leftAnchor).isActive = true
        button.rightAnchor.constraint(equalTo: view.rightAnchor).isActive = true
        button.topAnchor.constraint(equalTo: view.topAnchor).isActive = true
        button.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
    }
}