I have a concrete plant with 120volt e-stops on it, installed on hoffman metal boxes osha came out and asked if they where less than 50 volts to the e-stops do the e-stops have to be less then 50 volts. This plant was installed in the early 90's and inspected I never heard of this before.
OSHA always provides a specific list of violations. They don't just ask about whether something is under 50 volts. They either cite a violation or not. If they didn't, it's fine. If they did, you'll get a notice in the mail within a couple of weeks with everything spelled out.
To answer your question, OSHA considers anything under 50V to be non-hazardous. So if there are exposed energized parts or fuses that need to be changed hot or even light bulbs that need to be changed hot, or something that's not grounded, it's okay under 50 volts but not okay at 50V or more. That's the only 50V distinction I know of.
Get a code reference, as to why it is a violation. 120 volt controls are not a violation. Is it possible the E-Stops are in a classified location. I am certain that dust is an issue in concrete plants. Very well could be considered a classified location. But even in a classified location the enclosure would be the protecting means not the voltage.