Well, if you work for a state college/uni/other entity like I do, then the salary information is public. Former employers can say or do what they like, they rarely do anything other than verify dates of employment, confirm a few facts, out of a perceived fear of a civil suit, (I guess). Credit reporting agencies OTOH, and other similar companies use all sorts of information to build profiles about you / your habits / other demographic data, and I'd be surprised if there weren't some of these companies who would sell either predicted (and perhaps erroneous) salary range for you, or a factual (if partial) salary history mined from various sources.

>I wonder if you could just counter the request by saying you signed a NDA that forbid sharing that information.

Seems reasonable, or some version of "No thanks, I'd rather not."

> They say former employers can only verify dates of employment

I don't know who "they" is, but this isn't true at all. Many large-scale employers are only willing to verify dates of employment, because they're scared of being pulled into a defamation lawsuit. But that's just an overabundance of caution from them. There's nothing to stop your former employer from responding to requests with "southphillyman typically visits the toilet every day between 11:20 and 11:40".

Some countries publish salary / income for all citizens..