robots-rewrite.php

<?php
/**
 * Plugin name: Modification of robots.txt
 * Version:     0.1.0
 */

/**
 * Modify robots.txt for main site and english site
 *
 * @param $output
 * @param $public
 *
 * @return string
 */
function fbn_custom_robots( $output, $public ) {
	$site_id = get_current_blog_id();
	if ( $site_id == 1 ) {
		$output .= "Disallow: /agb-und-widerruf/n";
		$output .= "Disallow: /mein-konto/n";
		$output .= "Disallow: /warenkorb/n";
		$output .= "Disallow: /impressum-und-datenschutz/n";
	} elseif ( $site_id == 11 ) {
		$output .= "Disallow: /account/n";
		$output .= "Disallow: /cart/n";
		$output .= "Disallow: /imprint/n";
		$output .= "Disallow: /terms/n";
	}

	return $output;
}

add_filter( 'robots_txt', 'fbn_custom_robots', 20, 2 );