This is not going to be nearly as helpful as people think, as long as it's still legal for employers to verify salary once it's been revealed.

People who know or suspect that their salary is above the average applicant's will volunteer it to the prospective employer. Other people will choose not to disclose.

We will reach an equilibrium where non-disclosure will be an honest signal of lower current salary.

The main difference from the current state is that employers won't know the exactly salary of the lower non-disclosing applicants -- this helps people at the absolutely bottom of the 'non-disclosure' range, but hurts those who are just below the average salary. It should have no strong effect on people with above-average salaries who choose to disclose.

Of course, if employers aren't allowed to verify voluntarily disclosed salary, then many people will just lie.