Although I don't think the film actually dates any of the events in the third act, it does feel like it compresses time.
The US events were turn of the century, and Bolivia was some eight years later, although the film doesn't seem to give much indication of that.
But in either case, electricity was hardly unknown by then. I think the subtle nods too it were entirely deliberate.

Western Electric Company patented an automatic, electric fire alarm system in 1871for use in large buildings like hotels called an electro-mercurial annunciator. I'm pretty sure banks would have some sort of electrical alarm system in the late 1800s.

Must a been.just went back, and the scene where "Sweet Face" is supposed to throw the posse off & Butch and Sundance are in the prostitutes room, an ELECTRIC LAMP is clearly plugged into the wall

This was the 1890's or so and electricity was getting pretty widespread so that may have been real.

When the gang first gets off the train in Bolivia, as Sundance is walking up to the entrance-way, in the background, you'll see a MOTORCYCLE fly by!