As long as the payment you are receiving is attibutable to the work you performed in the state of California, then that payment is subject to CA state taxes, regardless of where you live at the time the payment is made to you.
Even though the severance pay is not actually for services you provided, it is directly related to the services you provided while with this company, and is therefore subject to CA tax.
These same rules regarding severance pay apply to all states, and not just to the state of California.
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Thank you patrolia