iOS13から、AppDelegateではなく、SceneDelegateに変わったので、ViewControllerのメンバ変数やメソッドにアクセスする方法を調べました。
基本は、SceneDelegateのメンバ変数windowのrootViewControllerから、目的のViewControllerを探します。
rootViewControllerがUINavigationControllerの場合、目的のViewControllerは配列の中に入っています。
それぞれ、目的のViewControllerのクラスにキャストするのを忘れないでください。
キャストすれば、目的のメンバにアクセスできます。

SceneDelegate.swift

class SceneDelegate: UIResponder, UIWindowSceneDelegate {
    var window: UIWindow?
    
    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        // Use this method to optionally configure and attach the UIWindow `window` to the provided UIWindowScene `scene`.
        // If using a storyboard, the `window` property will automatically be initialized and attached to the scene.
        // This delegate does not imply the connecting scene or session are new (see `application:configurationForConnectingSceneSession` instead).
        guard let _ = (scene as? UIWindowScene) else { return }
    }

    func sceneDidDisconnect(_ scene: UIScene) {
        // Called as the scene is being released by the system.
        // This occurs shortly after the scene enters the background, or when its session is discarded.
        // Release any resources associated with this scene that can be re-created the next time the scene connects.
        // The scene may re-connect later, as its session was not neccessarily discarded (see `application:didDiscardSceneSessions` instead).
    }

    func sceneDidBecomeActive(_ scene: UIScene) {
        // Called when the scene has moved from an inactive state to an active state.
        // Use this method to restart any tasks that were paused (or not yet started) when the scene was inactive.
    }

    func sceneWillResignActive(_ scene: UIScene) {
        // Called when the scene will move from an active state to an inactive state.
        // This may occur due to temporary interruptions (ex. an incoming phone call).
        print(#function)
        
        //rootViewControllerがUINavigationControllerか確認
        //UINavigationControllerを使ってなければ、ViewControllerとかでOK
        if self.window?.rootViewController is UINavigationController {
            let nc: UINavigationController = self.window!.rootViewController as! UINavigationController
            //ここでは、UINavigationControllerなので、ViewControllerはUINavigationControllerに内包されている
            let vc: ViewController = nc.viewControllers[0] as! ViewController
            //ここでメンバにアクセスしています
            if vc.blnProperty {
                //何らかの処理をする
            }
            
        }
        
    }
}