Higher salaries will get disclosed more, because if I get an offer below my current salary, then I can disclose and at the very least get a match or refuse the offer. At a lower salary, there is a higher probability that the offer is higher, and my current salary does not matter.
I believe that people more rarely accept a downgrade in salary, so the probably of acceptance is also conditioned on the current salary. What this means is that the effect is biased and probably non-linear, but I wouldn't qualify it as non effect.
