theMikeD

An excerpt is a (usually) brief, text-only post summary. It is used in various places of a theme when a summary of a post is needed, such as on your blog page and archive pages for terms.
Excerpts are either manually crafted or (usually poorly) automatically extracted from the main post content. Hand crafted ones are better because
<ul>
<li>they avoid things like sentences being cut off in the mid…</li>
<li>they allow you create an actual summary and not just the first x words from the post content</li>
<li>they allow you to do better SEO targeting</li>
<li>they look better</li>
</ul>
This plugin will enforce the use of hand-crafted excerpts by your users by preventing a post without an excerpt from being published.
Classic editor support is based on the work done by Scott Walkinshaw and other contributors, found <a href="https://gist.github.com/swalkinshaw/2695510" rel="nofollow ugc">here</a>.
<h3>Upgrading from a previous version</h3>
Use the upgrade link in the Dashboard (Dashboard > Updates) to upgrade this plugin.
<h3>License and Disclaimer</h3>
This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License 2 as published by 
the Free Software Foundation.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
The license for this software can be found here: <a href="http://www.gnu.org/licenses/gpl-2.0.html" rel="nofollow ugc">http://www.gnu.org/licenses/gpl-2.0.html</a>

Mandatory Excerpts for WordPress

Causes the excerpt to be required before a post can be published. Supports both editor types (classic and block aka gutenberg).

J0SiE

Created to help those people who don’t like to edit code in template files (or are too lazy to do that). 
Just activate the plugin and it will automatically show the_excerpts in your homepage, archives and search results. 
It tries to display your custom excerpt text and if it doesn’t find it it will show an automatically generated excerpt. 
You can also define excerpt length, custom read more link and attach a thumbnail.

Auto Excerpt everywhere

Thanks to author for creating this plugin. 
It's works out of box. And working fine at WordPress Version 4.6.1


Works out of the box on WP Version 4.6.1

The plugin automatically displays excerpts instead of full post contents in your blog. Custom excerpts and generated excerpts supported.

Mandatory Excerpts for WordPress vs Auto Excerpt everywhere