The tag on a news van has expired

"Tag has expired" means that it is currently unavailable, deprecated or has been deleted.

And what could be a tag on a car? Maybe a registration plate?

In the context of your original sentence, "The tag on a news van has expired", "Tag" would mean the license plate on a vehicle. "Expired" would then mean your (usually annual) vehicle registration/license plate had not been paid for by the due date and was no longer valid. As in, you had better get to a Department of Motor Vehicles office quickly, make payment, and get your car registration renew

It could also refer to the license plate (often referred to as the license tag) attached to the vehicle.
Local, state and/or national authorities often require the owner of a vehicle to pay a fee to keep the tag current. When you have paid the fee, a small sticker will be provided to affix to the license tag showing that you have paid this year's fee.