IIRC from the San Bernardino case, that was the case on the shooter's phone (a 5S I think), but since then it has been built in to the "Secure Enclave" chip which is not affected by the vulnerability. But you could flash a fake iOS that logged the user's code and/or directly exfiltrated the data--the user would have no idea that it was not the stock iOS they were giving their code to. It would leave them which a bricked device the next time they rebooted, which could raise suspicions.