Kristopher Lagraff

This plugin enables the short link menu item to display in the admin toolbar. This feature was removed from 
version 3.3 by default.

Reenable Shortlink Item In Admin Toolbar

This plugin will display the shortlink menu item in the admin toolbar.

Thorsten Frommen

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.

Bring Back the Get Shortlink Button

This plugin brings back the Get Shortlink button, which is hidden by default since WordPress 4.4.

Reenable Shortlink Item In Admin Toolbar vs Bring Back the Get Shortlink Button