Most discussions about salary are federally protected in the US and backed up by court decisions in nearly every state. If you get fired for talking about salary you almost certainly can seek redress from the National Labor Relations Board.
I've seen people get fired for being late to a meeting once. The real reason they got fired was something else but on paper and what they told the person was for being late.
I've also seen manager training materials that specifically tell managers not to even hint that salary is a secret topic as the liability involved is problematic.
It is one thing to claim that worker protections don't exist, its another to claim that you refuse to take advantage of them.