There are a lot of things one could do when wiring that seem like they'd be fine, or even actually are fine, but go against code. And reading the code, one really does get the impression that a lot of it was "written in blood".
Based on your wording, I don't know if you know this. But presently the issue of practical accessibility is actually moot thanks to the work of Malamud. The NEC, along with many others, is available right here: https://law.resource.org/pub/us/code/ibr/
i'm sure that the majority of the times i've been told that, there was a good basis written down someplace.
but not having access to the code further skews an already asymmetric relationship between domain expert/contractor and customer.