We'll create fresh WordPress site with Bring Back the Get Shortlink Button installed. You have 20 minutes to test the plugin after that site we'll be deleted.
As of WordPress 4.4, the Get Shortlink button is hidden by default. This plugin brings it back. Yay.
Please note that the button does not exist at all in a Block Editor context. It will only show up when using the Classic Editor, for example, when editing a single attachment.