Is discussing salary at work allowable? Yes. Even if you have a company policy against it? Yes.
In fact, having a policy against it could get you in hot water with the National Labor Relations Board (NLRB) because such policies generally violate federal labor law.
The National Labor Relations Act protects employees’ rights to discuss conditions of employment, such as safety and pay even if you’re a non-union employer. The NLRB calls these discussions “protected concerted activity” and defines them as when employees “take action for their mutual aid or protection regarding terms and conditions of employment.”