I spoke with a salesperson who told me he'd received a deposit on a 2022 C8 but the deal was cancelled after GM determined the perspective buyer was on its blacklist. I trust this salesperson. He had no reason to make this up. He was disappointed for that customer and sold the C8 to someone else.
When I bought my 2020 C8 I was told upfront of the "12,000 km AND 1 year rule" by my dealer and wasn't required to sign anything. I only sold my C8 after 14 months and 12,400 km on it, since I didn't want to risk not being able to buy a Z06.
My neighbour who purchased a 2021 C8 was also told of this same GM policy, by his salesperson and he similarly didn't have to sign anything.
Finally, I had a friend who bought a 2020 C8 and his dealership required he sign an agreement outlining the "12,000 km AND 1 year rule". I'd heard of others having to sign similar acknowledgements but I actually saw a copy of my friend's signed document.
As others have said, like it or not GM can set its own rules and has this policy, like other car manufacturers (so it's not simply GM that does this). The rule makes sense since car flippers can make a +++ handsome profit with the demand on the C8, combined with the US/Canada exchange rate...so they buy a C8 in Canada and sell it down in the States. The sale is helped by the fact that GM will honour the warranty on the car, down in the States. But the US dealers are unhappy finding a Canadian car (originally sold cheaper in Canada) in the States and knowing that they've been deprived of a sale and their profit.
Believe it or not. Up to you. You may think this is unfair. That's your choice. I'm assuming that all dealers should be telling their customers of this GM policy but like anything, likely not all of them do. The benefit of being on these Forums, is that everyone has the opportunity to know of this GM policy, whether or not it's believed to exist. So you can choose to modify your actions or not.