Simple and straightforward function to get the related posts by tags in wordpress

get_related_posts.php

<?php

/**
 * Return an WP_Query object with the posts related by tags
 *
 * Only works with posts that have tags
 *
 * @author  Felds Liscia    https://github.com/felds/
 * @param   integer|null    $max_posts      The number of posts to fetch
 * @param   integer|null    $post_id        The post ID
 * @param   array|null      $query_args     Extra query args (for sorting etc)
 * @return  WP_Query
 */
function get_related_posts($max_posts = 4, $post_id = null, $query_args = [])
{
    $post_id = $post_id ?: get_the_ID();
    $tags = get_the_terms($post_id, 'post_tag') ?: [];
    $tag_ids = array_merge(
      [0],
      array_map(function ($tag) { return $tag->term_id; }, $tags)
    );

    $args = array_merge($query_args, [
        'tag__in' => $tag_ids,
        'post__not_in' => [ $post_id ],
        'posts_per_page' => $max_posts,
        'ignore_sticky_posts' => true,
    ]);
    
    $query = new WP_Query($args);

    return $query;
}