The technical term for what you're describing is under-frequency load shedding (UFLS) and it is part of most modern power systems around the world, including all of the North American grids. It is an important safeguard against system collapse, however, the goal of system operators is to avoid reaching this stage. In fact, resolving contingency events without interruption of "firm load" (that is, loads that have not previously agreed to be interrupted) is an important performance metric for any grid operator.

UFLS is mostly intended to buffer transient loss of power supply (for instance, tripping of several large generators). If the grid operator knows they are facing an inadequacy of generation supply and all resources have already been called in, they will start to shed load under manual operator action to avoid UFLS activation. Before doing this they will declare an emergency which, generally speaking, requires all generators to make best efforts to supply as much energy as they can to the system.

Think of it like automated emergency braking on a car vs driver braking. AEB is great, but if you can already see that you're gonna hit something, just hit the brakes right now instead of waiting for AEB. By the time UFLS kicks in, you're already in dire straits and have only moments before reaching an unrecoverable state.