It's all very strange really how the broiler industry developed. Up through the 1950's, most meat birds in the US were excess males from laying flocks (Leghorn and a handfull of other 'purebreeds'). They were fed differently and grow better, but obviously the results were nowhere near as agressive as we see today.
However, when we say 'breeds' in chicken, what are we talking about? There are no chicken breeds. You cannot do a DNA test on an animal and say "yes, it is a Rhode Island Red." like you could with certain cattle. Even amongst Leghorns, there were regional differences in appearance and performance, always with an eye towards performance.
Individual farms had their own crosses they would try. Some would net them better layers, thus increasing their egg profits; but simultaneously would net them less money selling the cockrels as meat. Others went 'dual purpose' and had a balanced income. The chicken genome was far more diverse back then. So even if you were to say that "Buckeyes were used in Iowa historically" all Buckeyes were not equal and none probably matched the Standard of Perfection, to which 'breeds' are allegedly to adhere.
Today, things are consolidated into very large breeders selling breeding stock and hatching eggs to commercial productions. The system is streamlined and far more economic, but at the cost of diversity and the 'little guy' was squeezed out. It does mean that throgh online hatcheries, many more people can get into chickens and are exposed to more breeds... but, realize Murray McMurray, et al are likely buying their hatching eggs or breeding stock from the same farms and enthusiasts, so you are getting the same product regardless of whre you get your birds.
So, hertiage meat chickens? You can pick any breed you'd like and you could say with some certainty that at some point in time, those birds were eaten for meat. As stated above, the feeding and management of them will probably be on equal footing as the breed you select.
In the same breath, you could probalby let your heritage breeds crossbreed themselves, and still say with certainty that somewher at some point in time, those birds were eaten commercially.
A lot of people raise Dark Cornish as heritage meat birds as there is still a small market for these which are seen as delicacies. I raise them myself and yes they get big; but they're still no match for a broiler. I guess there is a flavor difference. As the birds get older, they grow closer to sexual maturity, and they gain a more robust texture and flavor (which I tend to like). So, if I were told I could only have 1 breed of chicken to eat, and it had to be purebred, the Dark Cornish would be my personal choice.
But, five years from now, your decision will be inconsequential.