California might consider making this sort of practice illegal, in the same way they've prohibited requiring a salary history, or enforcing certain non-compete agreements.

Why? It's salary discrimination based on a tangential factor that arguably has no effect on the quality-of-work delivered.

To the extent an employer has the power to demand such a disclosure, it's suggestive of market-concentration & a lack of competition, that prevents an employee's skills, and skills alone, from determining their compensation.

(And, a possible added bonus for California would be that requiring big employers to pay California salaries, even to non-California employees, might deter those employers from seeking such California-tax-base-eroding arrangements.)