We call it the 25-year rule. It's shorthand for a combination of regulations that allow you import any motor vehicle older than 25 years, assuming the car also meets your state's regulations. Specifically, the Environmental Protection Agency's 3520-1 form and the Department of Transportation's HS-7 form are required by Customs and Border Protection to bring a vehicle into the United States.
https://www.hagerty.com/media/news/seven-cars-you-can-finally-import/
https://www.hagerty.com/media/news/seven-cars-you-can-finally-import/
