wingedit.pl/wp-content/plugins/really-simple-ssl/security/wordpress/user-registration.php

<?php
defined('ABSPATH') or die();
/**
 * Action to disable user registration
 *
 * @return bool
 */
function rsssl_users_can_register($value, $option) {
	return false;
}
add_filter( "option_users_can_register", 'rsssl_users_can_register', 999, 2 );