agrafix

This plugin adds hyperlinks to specified words in posts and pages. This can be very usefull if you are using complicated words and want to link Wikipedia articles or 
if you just want to improve your seo by linking some interessting words 🙂
<h3>Example</h3>
Your Text: 
 Welcome to a blog about google
Your Plugin-Settings: 
 blog|Wordpress software|http://wordpress.com 
 google|Search engine|http://google.com
Result: 
 Welcome to a <a href="http://wordpress.com" title="Wordpress software" rel="nofollow ugc">blog</a> about <a href="http://google.com" title="Search engine" rel="nofollow ugc">google</a>

agLinker

This plugin adds links to specified words in posts and pages.

leocolomb

This plugin adds internal and external links into post and page, based on different sources:
<ul>
<li>Custom keywords</li>
<li>Posts</li>
<li>Pages</li>
<li>Categories</li>
<li>Tags</li>
</ul>
The plugin provides simple and standard interfaces:
<ul>
<li>Clean & clear admin page that allow highly customizable usage</li>
<li>Useful CLI commands</li>
<li>Respect PHP standards, web standards and security standards</li>
</ul>
Low load impact in mind, some requirements are requested:
<ul>
<li>PHP 7.2+ (⚠ requirement)</li>
<li>WordPress 5.0+ (⚠ requirement)</li>
<li>Object Cache handler (⚠ requirement)</li>
</ul>
<h3>Contributors</h3>
<a href="https://colombaro.fr" rel="nofollow ugc">Léo Colombaro</a>

agLinker vs Auto Links for WordPress