Hot Wheels Unleashed has been updated to version 1.07 today by Milestone. You can find out the complete patch notes for this update here.
The upcoming support for DLCs includes a Batman expansion that was announced back during the DC Fandome. It is scheduled to be launched tomorrow so if you have received an update today, it is probably related to adding support for this upcoming DLC. You can get the details on this update below, or have a look at the official website.