Because you can't predict offline which names you need to obscure with fake NSEC records.
If there is any company in the world actually using Phreebird in production, I'd --- for more than one reason --- like to know about it.
If there is any company in the world actually using Phreebird in production, I'd --- for more than one reason --- like to know about it.
(And indeed, it cannot be done offline - although doing much narrower NSEC/NSEC3 ranges than 'normal' could be done offline).