"Heirloom" was (and is) a compromised term made up to quantify "old time taste" by a tomato growing group pre-internet. It has no botanic meaning. "Open pollinated" is the term that will bring you home.
Much of what is most coveted by tomato growers in particular. Like colors other than pink-yellow-red simply weren't around before WW-II. And as a result also don't qualify as 'heirloom'.
