Michael Ott

It’s often too easy to make edits to your web site that result in a broken link. This plugin will automatically email the web site administrator (or a specified email address) whenever a 404 is encountered, along with a link to the page the 404 originated from. With this knowledge in hand, you can search the page for the problem and either remove or fix it.
Notifications are also triggered by missing media, such as images, video, CSS files and more.
<h3>Support</h3>
Report any issues at the official <a href="https://wordpress.org/support/plugin/404-notify/" rel="ugc">404 Notify Support</a> WordPress page.
<h3>How to use</h3>
Once activated, notifications are automatially sent to the site administrator, or, you can specify an alternative email address in Settings -> General and look for the 404 Notify email address field.
<h3>Uninstall</h3>
Deactivate the plugin, delete if desired.

404 Notify

It works as presented. When an internal link leads to 404 you get an email alert. 

Job well done

Get a notification whenever a 404 is encountered on your web site.

jidaikobo

Check accessibility of target page and generate accessibility evaluate page and policy.
“Accessibility policy” and “accessibility report” required by WCAG 2.0 and JIS X 8341-3:2016 can be created.
When installing the plug-in, you can automatically check individual posts when saving, so that you can constantly manage the site with accessibility awareness.
<a href="https://translate.wordpress.org/projects/wp-plugins/jwp-a11y" rel="nofollow ugc">translate by using GlotPress</a>
thx: 
* <a href="https://github.com/mustangostang/spyc" rel="nofollow ugc">spyc</a> 
* <a href="https://github.com/guzzle/guzzle" rel="nofollow ugc">guzzle</a> 
* <a href="https://plugintests.com/plugins/jwp-a11y" rel="nofollow ugc">PluginTests</a> 
* <a href="https://github.com" rel="nofollow ugc">GitHub</a> 
* <a href="https://www.owasp.org/index.php/OWASP_Zed_Attack_Proxy_Project" rel="nofollow ugc">OWASP ZAP</a>

jwp-a11y

Check accessibility of target page and generate accessibility evaluate page and policy.

404 Notify vs jwp-a11y