The frequency does drop too when the grid becomes unstable. This is long past the point of simple voltage drop. Who the heck remembers all the stuff they learned about system stability 30 years ago, unless they use it regularly? It hasn't changed though...lots easier to properly model now.

One of the results of the big 1965 NE power outage here was that "we" (speaking as if I was still a utility employee) and the U.S. agreed on a frequency detection system. What it does is also detect the rate of change of frequency, much better than just detecting absolute frequency, faster detection of probs. Various parts of the power system within the range of each detector are dumped, depending on how fast the freq is dropping (usually). Every transformer station or larger would have at least one of these, so there are a lot of them and almost everywhere populated would have many.