Not quite, at least nationally in the US. The National Labor Relations Act only bans prohibiting employees from discussing salary with other employees of the same company. Prohibitions on discussing salary with people outside the company are fine.

Also, it does not cover contractors or supervisors.

Some states may have more extensive prohibitions.

A) A recruiter isn't necessarily going to know that. B) Even if they did, companies do illegal things all the time and get away with it. Maybe it's technically illegal, but also maybe your company would come after you for something trivial and just ruin your life with litigation. I totally understand actually wanting to abide by an NDA even if it's unenforceable.