Note: Some plugins are not yet compatible with PHP 8 or WP 5.7 and are therefore deactivated
Showing related posts is a common feature in many themes, but not all. With Related Posts by Taxonomy, you can add related posts to a sidebar, widget area or footer, using shortcodes and hooks, with very granular control over included terms and posts.
The following code, shows five related posts, from the taxonomy id 3 which is this case is the HTML Valid tag.
[related_posts_by_tax include_terms="3" posts_per_page="5"]
Extract from WordPress.org
This plugin is capable of finding related posts in multiple taxonomies and post types. Include or exclude terms from the search for related posts. Change the look and feel by using your own templates in a (child) theme.https://wordpress.org/plugins/related-posts-by-taxonomy/