11/6/2018 The lastest way

It is the best way to hide left back title of the navigation bar so far.

class YGCustomNavigationVC: UINavigationController {
    override func viewDidLoad() {
        super.viewDidLoad()
        self.delegate = self
    }
}

extension YGCustomNaviVC:UINavigationControllerDelegate {
    func navigationController(_ navigationController: UINavigationController, willShow viewController: UIViewController, animated: Bool) {
        let item = UIBarButtonItem(title: " ", style: .plain, target: nil, action: nil)
        viewController.navigationItem.backBarButtonItem = item
    }
}

6/5/2018更新

iOS 全局修改UINavigationBar的返回按钮（适配 iOS 11）

This way is no pefect when A viewcontroller which has hidden navigation bar push to B viewcontroller which will show it.
It won't hide left back title of the navigation bar in iOS11

3/2/2018更新

The following code do work perfectly before iOS11, If you want hide back title of the navigation bar.

UIBarButtonItem.appearance()
.setBackButtonTitlePositionAdjustment(UIOffsetMake(0,-300),  for:UIBarMetrics.default)