robots.txt is published
robots.txt is the first file most crawlers (agent or otherwise) request. Even if its rules are permissive, having it tells well-behaved bots they're at the right host and lets you point them at sitemaps.
How the check decides
The check sends GET /robots.txt. It passes if the response status is 2xx. It fails if the file is missing.
How to implement it
Serve a robots.txt at the root of your domain. At minimum it should declare User-agent: * and either explicitly Allow: / or list the parts of your site you don’t want indexed.
Pass
User-agent: *
Allow: /
Sitemap: https://example.com/sitemap.xml
Fail
HTTP/1.1 404 Not Found