From my somewhat limited knowledge of foraging the biggest precaution to take with ramps is not over harvesting the patch you are picking from. Denuding the soil and removing a majority of the bulb and root mass will prevent more ramps from coming up in the future. Some will suggest harvesting no more than 10% of any given patch is sustainable, others stay away from harvesting the bulb altogether, generally avoiding harvesting young ramps is good practice.
What park was this sign in? I am actually surprised foraging is a common enough activity for the state to put up a sign to stop people. Maybe there was a pandemic boom of foraging like so many other outdoor activities? Ramps have a bit of a lower barrier to entry than say mushrooms so I guess it's not completely surprising a sign just pertaining to ramps. Most hardcore foragers have their secret honey holes off the beaten path so I imagine this sign was in a well trafficked area?