Yes, that was exactly the kind of thing I had in mind. Old-fashioned TVs depended on this, too, IIRC, and probably a lot of other stuff. (Newer TVs might still depend on it, for all I know. I haven't really looked into the matter.)

As I recall, the original goal was to try and maintain a solid 60Hz at all times. Then that was relaxed to trying to maintain an average of 60Hz over a 24-hour period. And I think that requirement has now mostly gone away, too, which is a good thing because with so many renewables in the mix these days you probably can't really do it anyway. But you can't let it slip too far because then you still might run into any number of operational problems.