Despite the URL using "200" and not "2xx", any status between 200-299 inclusive would flag the server as having unreliable status codes. https://w3c.github.io/webappsec-change-password-url/response...

I have my webserver return the /index.html status 200 for any non-existent path in /. Since I didn't want bots to be able to detect I was doing this by this method I set nginx to return a 404 instead just for that path.

    location /.well-known/resource-that-should-not-exist-whose-status-code-should-not-be-200 {
        return 404;
    }