I'm not sure about other utility companies, but when I was a TSO we never did this. For the most part, AGC worked fine to maintain frequency.
General Topic: http://www.pnnl.gov/main/publications/external/technical_rep... http://www.sciencedirect.com/science/article/pii/S0196890416...
HVAC Demand/Response: http://ieeexplore.ieee.org/document/7407597/?reload=true
Aluminum Smelter Demand Response: http://repository.cmu.edu/cgi/viewcontent.cgi?article=1836&c...
> "Autonomous Demand Response for frequency regulation is real..."
I wasn't saying that ADR wasn't "real," just that it's not something we're currently doing for frequency control in actual practice...and of course that might change as engineers continue researching and pushing the field further. It definitely looks like ADR solutions are on the horizon though.
In the US frequency response is mostly Nat Gas powered. There are new projects that are battery based, but another way to achieve this is on the demand side, by requesting non critical loads to turn off or throttle themselves when the frequency drops too low. I think this is being done at an industrial scale (i.e aluminium smelters) but it is possible on the domestic scale also with EVs electric tank water heaters, etc.
Going forward, given the intermittency of renewable energy, these methods (primarily storage) will be more important as the energy supply becomes more variable.