But what do you mean by "online signing" ?
(I'm not sure if this is what was being asked, but I'm curious about the answer to that, either way.)
(Phreebird is an online signer, but I don't see why an offline signer couldn't generate these proofs.)
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).