robots-txt.php

+<?php
+/*
+ * Plugin Name: A/I - Robots.txt
+ * Description: Customize robots.txt with per-site sitemap links
+ * Version: 0.0.1
+ * Author: Autistici/Inventati
+ * Author URI: https://autistici.org
+ */
+
+function ai_robots_txt_add_sitemap($output, $public) {
+    if ($public != '0') {
+        $sitemap_url = get_site_url(null, "/wp-sitemap.xml");
+        $output .= "\nSitemap: {$sitemap_url}\n";
+    }
+    return $output;
+}
+
+add_filter('robots_txt', 'ai_robots_txt_add_sitemap', 99, 2);