I only did declines when I first started training in the belief that it would actually develop the "lower chest". After it was explained to me it was total bullshit. I stopped. Haven't directly trained "lower chest" in 35 years. It's right up there with training "lower biceps". My chest turned out fine..
The whole point of decline pressing is reduce the involvement of the deltoids in the movement, which I find to hold true.
I wouldn't say that upper chest training is a myth per se, but I think that any chest movement that adequately stimulates the muscle for an individual will hit the whole muscle, upper & lower