You can junk a car with an expired registration in California.
You only need to have the vehicle registered in California to drive it down the road.
But when junking or scrapping the vehicle in California you don't need it to be registered.
In order to junk or scrap a vehicle in California and other states you only require the title in your name.
As long as the title is in your name you can take the vehicle to the scrap yard and sell it for scrap metal.
Without the title though the scrap yard won't accept it because law requires the scrap yards or anyone else buying scrap vehicles for junk to require the title be given to them as well upon the sell of the scrap vehicle.
Also according to California law and California DMV.
Prior to dismantling a vehicle, the vehicle owner must submit an application to DMV to record the vehicle as “Junk.”
When an owner dismantles a vehicle prior to doing this, the owner is subject to an investigative service fee.
The following must be submitted which is the evidence of ownership for the vehicle.