Not necessarily no.
What I think they would do is to start barring the registrations of the oldest/dirtiest vehicles first and then work their way up.
pre emissions would get told they can't be registered first, then pre-injection, then obd1 vehicles, then obd2. This could be over the course of 20-25 years.. that would give the populace PLENTY of time to adjust and either buy a newer ICE vehicle that will still be legal for how many ever years it could be or do the 'right thing' and buy an EV. They could say people can still register a gas ICE vehicle as long it's some sort of hybrid..
As Ron has said, the average age of a vehicle on the road in this country is 12 or 13 years..
Going off of that.. I'd day California could probably immideately ban the registration and use of pre-injected vehicles and it wouldn't be an inconvenience to any sort of meaningful amount of its population.
The people who run this country are mongoloids at best. I'm standing firm on that 5k dollar bet.
The bet isn't that ALL gas powered vehicles will be outlawed, but they will definitely be restricted in one way or another..