Load shedding uses multiple parameters, of which frequency is one. As Greg mentioned, actual load values are few and far between when initially setting up a load shed scheme. However, certain large-scale (% of max load) changes are known to disrupt system stability and can thus be coded into the scheme from the outset (for example, a 50% step in total load vs current spinning reserve). Smaller changes will cause frequency perturbations that can be picked up through normal system monitoring.
Converting energy to motion for more than half a century