Create subsite when the site name is reserved in multisite

code.php

<?php

add_filter( 'wpmu_validate_blog_signup', 'wpmu_validate_blog_signup_cb' );
function wpmu_validate_blog_signup_cb( $result ) {
    $current_site = get_current_site();
    if ( ! domain_exists( $result['domain'], $result['path'], $current_site->id) ) {
        if ( username_exists( $result['blogname'] ) ) {
            if ( ! is_object( $result['user'] ) || ( is_object($result['user']) && ( $result['user']->user_login != $blogname ) ) ) {
                $errors = $result['errors'];
                $errors->remove( 'blogname' );
                $result['errors'] = $errors;
            }
        }
    }
    return $result;
}