You are correct except you left out that is cars/trucks build '66 and later, anything earlier didn't have any requirements if memory serves.
I don't think I've ever heard of my friends/family being hassled with it, even in uber liberal California.
I get pulled over from time to time and its usually curious police wondering why my Z doesn't sound like a I6. I am polite and usually we end up talking for a few mins about cars and I am on my way.
I think you'd have a hard time finding a LEO that is out to prove something to really get nailed for it.
But, as Murphy's Law states: "What can go wrong, will go wrong."