Dogan Ucar

WP Last Modified adds the ‚last_modified_date‘ shortcode to your WordPress installation. This shortcode shows the last timestamp of your post/page. Simply use the „format“ attribute for custom date formats (it uses PHP’s date() function). 
The “description” attribute enables a brief description which changes has been made with the last modification. 
WP Last Modified shows also the actual revision of and the publication date of the post/page. 
You can customize the text which will be shown under each post/page under settings -> WP Last Modified Settings. You can also hide one of the elements if you do not want them.
With version 1.1.0, WP Open Last Modified was renamed to WP Last Modified.
Features:
<ul>
<li>last modification timestamp of a post/page</li>
<li>publication timestamp of a post/page</li>
<li>timestamp format customizable</li>
<li>revision count</li>
<li>custom text with placeholders</li>
<li>last modification description</li>
<li>excluding single posts/pages from displaying the last modification timestamp</li>
</ul>

WP Last Modified

This plugins adds the last modified date, current revision and the publication date of your post/page. Simply use the shortcode [last_modified_date]

Evan Mattson

This plugin adds information to the admin interface about when each post/page was last modified (including custom post types!).
Enhanced areas:
<ol>
<li>Page/post admin tables – added <code>Last Modified</code> column which is also sortable.</li>
<li>Page/post edit screen (<code>post.php</code>) – added <code>Last modified on: *timestamp*</code> to <code>Publish</code> meta box.</li>
<li>Admin messages after editing a page/post – ie: <code>Post updated. *timestamp* View Post</code>,</li>
</ol>
No options currently available, but the output can be fully customized with filters and the shortcode can be easily customized using attributes!
<h3>Gutenberg, WordPress 5, and Beyond</h3>
This plugin does not yet enhance the new editor provided by Gutenberg and introduced as the default editor in WordPress 5.0. No plans exist to add support for this although it may be added in the future. 
Other areas of wp-admin enhanced by the plugin still work, as does the classic editor.

Last Modified Timestamp

Adds the last modified time to the admin interface as well as a [last-modified] shortcode to use on the front-end.

WP Last Modified vs Last Modified Timestamp