This is a very contentious subject. There has to be a negotiation at some point and employees have to share what they are willing to accept and the employer has to share what they are willing to pay. There is some validity to the idea that an employee is worth whatever they can get an employer to pay them, so their previous salary is an indication or ballpark of what that amount might be.

I think a less infringing question is for the employer to ask for the salary the employee is expecting. Then there is no lying and the employee can try for a 20% jump or more and see what happens. They can also just ask for the employer's offer but in practice the employer is going to go up slowly and find the lowest salary they would take anyway.