This option has been removed and is no longer available in the SharePoint 2013/2016.
Luckily there’s still a way to sign in as a different user in SharePoint 2013/2016.
Just go to the following URL:
http://<yourservername>/_layouts/closeConnection.aspx?loginasanotheruser=trueAs soon as you hit this page, the familiar pop-up will prompt you for the username to log in with. Once you log in, you will be redirected to the home page.
Method 2: (easy!)
To make your life easy further, I have developed a Google Chrome Extension. Once you add this to Chrome browser, you can sign in as a different user with a button click, and you can forget above URL.
Here is the link for the google chrome extension:
Add this extension to your chrome browser and please rate the extension to support me.