I created that particular error-message in varnish cache 15 years ago.

Max has many good points about error messages in general, but all of them require access to out of band information.

We in the Varnish Cache Project do not have access to that information, we dont know who runs the varnish instance or what kind of information they serve to what kind of clients.

This is why the default '503 message only exposes the "XID" nonce: That allows the administrator of this cache instance to find all the details in the log files.

Varnish Cache users who want to present something else can do that from VCL, and I'm pretty sure Fastly normally does.

But when all else fails, and here it must have, Varnish Cache errs on the side of caution.