I've heard that this social taboo on disclosing salaries is merely successful brain washing by the anti union movements of the early 20th century.
A lot of 18th century literature talks about money and how much a person had "settled" on them per year. Jane Austen and similar writers focused on this a lot. Jane's characters always knew how much every person made and how much they could expect to have if they married that person.
I am not sure how accurately that translated into real life though. Or how much it happened in the middle and lower classes, since most literature focused on the richer classes.
Source on the brainwashing idea? I've never heard it before.