It does appear to be operator-specific; I'm with Spark in NZ and my iPhone 6 already has an option under 8.0.2 to switch between 3G and 4G (there's no 2G option, but that makes sense since Spark doesn't have a 2G network). Does the option not show up at all in 8.0.2 on yours?
Yeah this is highly carrier-specific. I don't think you'd see it in the US. I don't see it with Rogers in Canada and Rogers is almost identical to AT&T in infrastructure.
I swear, this should be a sticky topic it comes up so often. The availability of the 2G/3G/4G toggle is controlled by the carrier settings file. It has always been this way.
It's going to be entirely different depending on carrier used. It would be nice if it wasn't, but that is how Apple implemented it. iOS 8.1 does not change this.