The word gets misapplied a bit, so watch out for that. Most hatchery pages describe their various breeds and often say, "this is a heritage breed". OK, yes, the breed has heritage, but the actual birds the hatchery is selling almost never is. See the cuteness of the marketing there?
True heritage birds are often rare, raised only by those dedicated to the breed and bred up to the SOP only by those knowledgeable to do so. A quality fowl will spiral downward in just a few generations of careless breeding. Just the nature of nature.
Here at BYC, there are many quality people doing awesome work in preserving and breeding the heritage fowl. Lots of threads, many dedicated to Large Fowl Heritage birds and individual, by breed, threads on the heritage birds. Do your homework. Like all things chicken, there are all kinds of folks purporting to breed and sell heritage fowl but the true is that the birds are merely hatchery stock being re-bred. To me, there is a lot of murkiness in all this. Reputation of the breeders is just about everything.
A true, heritage, sloooooooow growing, slow maturing bird will simply cost more. Cost more to buy. Cost much more to feed. The rewards come in being part of a preservationist activity and the stunning enjoyment of keeping such birds. This isn't for everyone. Production Barred Rocks will often feather out in 6-8 weeks, begin laying at 19 weeks, and lay 280 egg their pullet year. A true heritage Barred Rock will take twice as long to feather, half again as long to reach POL, and lay many fewer eggs. In exchange, the birds will be, or should be, much larger. 2 pounds larger in many cases. The true heritage fowl will be able to reproduce and brood their young. Again, this can be seen as a benefit of keeping the real deal. They should live longer productive lives. The personality and looks of the heritage birds can often take your breath away, especially after having become accustomed to only seeing hatchery stock all the time.
I hope this helps in your thought process.