2014 is when (by executive order) it was extended to employees of Federal contractors that weren't covered by the NLRA. For those covered by the NLRA (nearly all private companies) the law has been on the books for well over half a century[1] (discussing pay is considered a "concerted activity"[2]).

[1] https://www.law.cornell.edu/uscode/text/29/157

[2] http://www.govdocs.com/can-employees-discuss-pay-salaries/