Here’s a nice article on the code issue, which is nationwide in the US (it’s part of the NEC). It speaks specifically about the generator requirements:
TLDR: The NEC is against plugged-in furnaces for a good reason: they consume so much electricity that they need a dedicated circuit. When you have a plug-in furnace, you’re likely to plug it into a shared outlet with other appliances. As a result, the circuit gets overloaded, damaging the furnace and the other plugged-in electrical devices or even resulting in a fire.So, converting a hardwired furnace into a plugged-in constitutes an NEC code violation.
The article then proceeds to show how to safely violate the NEC with a plug and socket, and how to follow the letter of the law with a transfer switch or interlock kit.
I think these are the same devices my brother Peter might be talking about. :)