MotoPress

This plugin allows you to optionally keep the last post modified date unchanged when you update the post in WordPress Classic Editor.
It adds datepicker and checkbox to the Publish panel of every post. If you select checkbox when you update the post, the last modified date will change; if not, this date will not change no matter how many times you update the post.
This is a helpful thing if you make only slight changes to the post and don’t want it to be noticed by the search engine crawlers or even in the post information publicly visible on your website.
Only use the plugin to keep the last modified date unchanged for minor changes. If you make a major change to a post, make sure to check the box to update the last modified date, thus making search engines aware of the update.
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<h3>Copyright</h3>
Сontrol Post Modified Date, Copyright (C) 2023, MotoPress https://motopress.com/ 
Сontrol Post Modified Date plugin is distributed under the terms of the GNU GPL.

Сontrol Post Modified Date

Keep the last modified date unchanged for minor post updates.

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.

Сontrol Post Modified Date vs Last Modified Timestamp