Among the numerous changes in WordPress 4.4 with an end goal to clean the interface, the Get Shortlink button is hidden from view. In the event that you want to restore the button, take a look the Shorter Links plugin developed by Rob Allen. It gives users the ability to use custom text that overrides the default URL in addition to restoring the Get Shortlink button. However, in case you all need is the button without additional functionality, the Bring Back The Get Shortlink Button plugin by Thorsten Frommen is an option with a descriptive name. Get Shortlink is a decent illustration of WordPress hiding functionality without removing it which allows developers to create plugins like those mentioned above.
To restore the button without using a plugin, check out here to add the following code to your theme’s functions.php file.