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(Phreebird is an online signer, but I don't see why an offline signer couldn't generate these proofs.)

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.

I don't know of any either, but there are plenty running PowerDNS in online 'white lies' signing mode. And then, of course, there is Cloudflare.

(And indeed, it cannot be done offline - although doing much narrower NSEC/NSEC3 ranges than 'normal' could be done offline).