Georgi Popov

This plugin provides a list of similar / related entries to an article or a custom post type entry.
This list could be added in the form of a:
<ol>
<li>Gutenberg Block</li>
<li>Shortcode</li>
<li>Widget</li>
</ol>
If you add a widget to the sidebar it will only appear when on the details view of an entry, as similarity cannot be calculated when on a list page of multiple entries.
The syntax for the shortcode is <code>[magadanski-similar-posts id="" post_type="" taxonomy="" limit=""]</code>
You can use the attributes as follows:
<ol>
<li>ID — (integer) the ID of the post you’d like to get similar posts for. Default value: the current post ID.</li>
<li>post_type — (string) the post type you’d like to query. Default value: “post”.</li>
<li>taxonomy — (string) the taxonomy based on which you’d like to get similar items. Default value: “category”.</li>
<li>limit — (integer) a maximum number of results you’d like to receive. Default value: 5.</li>
</ol>
The similarity is considered based on the number of common categories (or other taxonomy terms) for this post and the rest of the posts in your blog.

M Similar Posts

This plugin lists similar to the current post based on the number of common categories.

cfischer83

Does your website utilize categories and tags? Does it use custom taxonomies? If so, this plugin will find similar content 
based on all your taxonomies. There are two ways to show related posts within your page.
<ul>
<li>The first way to show related content on your post is to use the widget provided. This only works when is_single() is true</li>
<li>The second way to show similar content on your site is to use the pk_related_return($post->ID); function which can be 
called programmatically anywhere you wish!</li>
</ul>
The Widget included with this plugin gives you the option to limit the amount of posts; it allows you to determine which 
fields to show: Featured Image, Author, Date, and Excerpt (Title is required); it allows you to determine which 
variant of the featured image to show: thumbnail, medium, large, or full. As of version 2.0, you can now decide whether the 
‘similar posts’ sorting prefers posts that are newer or posts that were created closer to the date of the post you’re 
viewing.
If you find the Widget doesn’t meet your needs or is too limiting, you can call the functionality programmatically using 
this function:
<pre><code>pk_related_return($post->ID, $args);
</code></pre>
Where $post->ID is the ID of the post for which you are wanting to show related articles.
The $args parameter is an array with the following values available to you (more coming soon):
<ul>
<li>posts_per_page (int defaults to 5)</li>
<li>thumbnail_size (string consisting of one of these values: “thumbnail”, “medium”, “large”, “full”. Defaults to thumbnail).</li>
<li>sort_prefer (string consisting of one of these values: “newest”, “closest”. Defaults to newest).</li>
</ul>
An example might be:
<pre><code><?php
$args = array (
 'posts_per_page' => 6,
 'thumbnail_size' => 'medium',
 'sort_prefer' => 'closest'
);
</code></pre>
The return value of pk_related_return is an array of objects that includes most of the fields within WordPress’s posts 
table plus permalink and featured image.
Future Additions:
Allow the user to specify only certain content types (posts, pages, custom) in a request. This would allow you to specify 
only products get returned, or only blog posts. This would only be an issue if content types share taxonomies.

M Similar Posts vs Similar Posts Ontology