Related posts by shared taxonomy terms

<?php
/**
 * Related posts.
 *
 * @package Acme_Theme
 */

defined( 'ABSPATH' ) || exit;

/** @return int[] IDs of related posts, best match first. */
function acme_related_post_ids( int $post_id, int $limit = 4 ): array {
	$key = "acme_related_{$post_id}_{$limit}";
	$hit = get_transient( $key );
	if ( is_array( $hit ) ) {
		return $hit;
	}

	$ids  = array();
	$tags = wp_get_post_tags( $post_id, array( 'fields' => 'ids' ) );

	if ( $tags ) {
		// Fetch extra candidates, then rank them by how many tags they share.
		$candidates = get_posts( array(
			'post__not_in'   => array( $post_id ),
			'tag__in'        => $tags,
			'posts_per_page' => $limit * 4,
			'fields'         => 'ids',
			'no_found_rows'  => true,
		) );
		$scores = array();
		foreach ( $candidates as $id ) {
			$scores[ $id ] = count( array_intersect( $tags, wp_get_post_tags( $id, array( 'fields' => 'ids' ) ) ) );
		}
		arsort( $scores );
		$ids = array_slice( array_keys( $scores ), 0, $limit );
	}

	if ( count( $ids ) < $limit ) {
		$cats = wp_get_post_categories( $post_id );
		$more = get_posts( array(
			'post__not_in'   => array_merge( array( $post_id ), $ids ),
			'category__in'   => $cats,
			'posts_per_page' => $limit - count( $ids ),
			'fields'         => 'ids',
			'no_found_rows'  => true,
		) );
		$ids = array_merge( $ids, $more );
	}

	set_transient( $key, $ids, 6 * HOUR_IN_SECONDS );
	return $ids;
}

// Clear caches when any post changes (simple and safe for small sites).
add_action( 'save_post', static function (): void {
	global $wpdb;
	$wpdb->query( "DELETE FROM {$wpdb->options} WHERE option_name LIKE '\_transient\_acme\_related\_%' OR option_name LIKE '\_transient\_timeout\_acme\_related\_%'" ); // phpcs:ignore WordPress.DB.DirectDatabaseQuery
} );

function acme_related_posts( int $limit = 4 ): void {
	$ids = acme_related_post_ids( get_the_ID(), $limit );
	if ( ! $ids ) {
		return;
	}
	echo '<aside class="acme-related"><h2>' . esc_html__( 'Related reading', 'acme-theme' ) . '</h2><ul>';
	foreach ( $ids as $id ) {
		echo '<li><a href="' . esc_url( get_permalink( $id ) ) . '">' . esc_html( get_the_title( $id ) ) . '</a></li>';
	}
	echo '</ul></aside>';
}