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.

gentlesource

This plugin creates a short URL from the blog post permalink and stores it 
in the database. The URL is displayed below the blog post along with a link 
to twitter that passes the short URL on.
You can use all short URL services that offer an API. Here are a few:
<ul>
<li>http://melt.li/</li>
<li>http://lin.io/</li>
<li>http://unrelo.com/</li>
<li>http://bit.ly/</li>
<li>http://u.nu/</li>
<li>http://tinyurl.com/</li>
</ul>

GentleSource Short URL

Automatically shortens the blog post URL.

Bring Back the Get Shortlink Button vs GentleSource Short URL