Sidenote: most companies in most countries do this. They tend to align on a standard set of salary grills for the region, that are more or less the same between each other, to keep competition, wage increases and employee churn to the minimum.

This is of course illegal in most places, but to counter this in court you need a whistleblower willing to go public and expose them with hard evidence, but since the workers actively involved in these wage fixing schemes are very well compensated along with NDAs, they have no reason to throw all that away in exchange for no other company hiring them ever again and your ex employer suing your for breaching your NDA. Sure, you might eventually win in court, but do you really want to drag your family through expensive lawsuits against mega-corporations who spend more on toilet paper than you can on a lawyer?