Chris Roberts

Annie provides comprehensive annotation options, including footnotes and highlighting. If Tippy is installed, footnotes can be displayed inline using tooltips. Also, footnotes are placed at the end of the post. For highlighting, you are able to use default highlighting styles or define your own.

Annie

Provides comprehensive annotation tools for WordPress posts.

Corrado Franco

This plugin allows one to create footnotes by using markdown notation. 
It is for footnotes only. No other markdown tag is taken into account
Example:
<pre><code>I have more [^1] to say up here.

[^1]: To say down here.
</code></pre>
If used with jetpack and enabled markdown, it only shows the buttons without modifying the text content. 
Unlike jetpack-markdown, footnotes will appear in the exact point where they have been inserted. To go back to text mode click on the footnote number.
N.B 
This plugin also works with Gutenberg. But the button only appears in the Classic Editor. 
For now with Gutenberg enter the codes directly.
<h3>Configuration</h3>
No configuration is necessary.
Considering that this plugin, like others working with “markdown”, parsing the page every time I load it, I highly recommend using a caching plugin.

Blank Footnotes

Simple plugin to show footnotes using markdown notation.

Annie vs Blank Footnotes