Anything like that is arbitrary and subject to interpretation. I personally don't think any chicken is "heritage" unless your own ancestors raised them. Gary Underwood, for example, would have heritage Rhode Island Reds since his dad was raising them since the 1920's. My Ameraucanas are not "heritage" because my immediate ancestors wanted nothing to do with poultry and would not have known what an Ameraucana was in their day.
So those birds many folks are calling "heritage" are in my mind just old breeds. It is an overused term. Of course that is just one person's opinion.