No, it means you have a car the state values at $500, you pay a small percentage of that as tax. Probably not much, but I don't know the actual tax rate in CT. It does not mean you pay $500 a year in tax on the car, or $2500 if they raise the rate. In states with ad valorum or personal property tax, the price you pay each year to register and tag a car declines with the car's assessed value. It can be a steep bill on a new car (paid $600for tags on a $45k dodge one ton back in 2004 in Alabama), but it drops quickly.
If people in CT are getting antique plates for run of the mill daily drivers that are 1991 or older cars, it's because the $500 assessed value is lower than what the book calls for car with regular plates.
However, I'm surprised boats don't get a personal property tax as well.