I have been told this formally only once. Every other time was an informal meeting over dinner or at a bar, or being told some nebulous statement like “that’s not really part of our culture here” when a manager discovered that I would freely discuss pay with coworkers.

It is explicitly illegal to require employees to not discuss pay. Of course you’re going to find that most people who were instructed to not discuss it felt “discouraged” and can’t point to some paperwork where it was official