Don't index WP sites with staging in the URL.

no_index.php

<?php
/*
Plugin Name: United Influencers Stop Indexing On Staging
Plugin URI: https://hmn.md/
Description: This plugin prevents search engines from indexing our staging sites.
Author: Human Made Limited
Version: 0.1
Author URI: https://hmn.md/
 */

namespace United_Bloggers\Stop_Indexing_On_Staging;

/**
 * Make sure the top level domain and subdomains are never indexed.
 *
 * @return string
 */
function stop_indexing( $blog_public_option ) {

	$server_name = $_SERVER['HTTP_HOST'];

	if ( strpos( $server_name, 'staging.' ) !== false ) {
		return '0';
	}

	return $blog_public_option;
}

add_filter( 'pre_option_blog_public', __NAMESPACE__ . '\\stop_indexing' );