Hostnames are not private information. There are too many ways how they get leaked to the outside world.

It can be useful to hide a private service behind a URL that isn't easy to guess (less attack surfaces, because a lot of attackers can't find the service). But it needs to be inside the URL path, not the hostname.

  bad: my-hidden-fileservice-007-abc123.example.com/
  good: fileservice.example.com/my-hidden-service-007-abc123/

In the first example the name is leaked with DNS queries, TLS certificates and many other possibilities. In the second example the secret path is only transmitted via HTTPS and doesn't leak as easy.