Horse is dead.
But it is a bit of a gray area.
Most 1970's cars only had a 5-digit odometer. However, that is valid for the laws that require a working odometer - it still works, you just have no way of knowing if the car as 45K miles, 145K miles, 245K miles, etc.
Then again - the burden of the law is on the owner. i.e. the law states the car must have a working odometer, working headlights, working turn signals, etc. If Toyota decides that the turn signals will only work for 50K miles and then you get a warning at 45K miles and have to buy new turn signals - I think they can legally do that. They should disclose that somewhere, and the issue I have with the Corolla is that if the odometer shows six-digits, it would be expected that it would work to 999,999 miles (km) and probably beyond. I would have less issue if it rolled over at 300K miles/kilometers, and really zero issue if they sold the car with a statement in the owner's manual that "The odometer will freeze when you reach 299,999 miles", but I'm not sure Toyota knew that.
Huh - it's not even related. The 2003 Corolla would be smog exempt in 2029. Doesn't matter if it has 100K miles on the odometer, 10K miles on the original odometer, or 10K miles on a replacement odometer, or if it has 299,999 miles on the odometer and has had that mileage for the past 10-years.
It is an issue for the seller with CarFax records and potentially an issue for safety inspections - although I don't know that any of them make you drive the car for a mile to make sure the odometer changes over.
And it could be an issue for smog states if any of them have an exemption above a certain mileage - i.e. if the state law says cars above 150K don't have to meet smog, how do you prove that your car had the odometer replaced 10K miles ago and your car REALLY has 310K miles on it.