Long ago before the internet people joined special interest clubs by mail. A several year discourse (according to Carolyn Male) between tomato-club members about the search for, "that old time tomato taste", led the club to adopt the term "heirloom" and to support the definition that, that kind of seed being saved for at least fifty years. Mm, now that established definition is now itself fourty years old and has had several interpretations.
What is isn't, is a routinely accepted botanic term used by seed houses. So when your being sold an item as a heirloom you do want to understand what the seed house thinks it is.
Any heirloom (or open pollinated) seed isn't, is in its first generation of being collected. When you see routinely used botanic terms like F-1 it is a hybrid. Hybreed plants have distinct properties inbreed.
F1 seed are not bad, but they are built for a reason, often with tomato, it is shipping stability. Or, resistance to known disease. They are breed the old fashioned way by selection of pollen and stamen.
GMO's have other genetic material inserted into the plasm. And are patented and not available through retail sales. GMO's are not F-1 hybrid.
If I lived in the deep south and was plagued by rootknot nematode, I might be grateful for plants with F-1 resistance built in.
But, and here is where the rubber hits the road for me, those old tomato or corn taste different. I cherish that difference. I will save or barter seed for that old time taste.
New open pollinated vegetables are still being breed. Some are pretty darned good. A few I save seed of after trial. Others not so much. Heck not every 'heirloom' is that yummy to me and I don't save those seed either.
Grow some, save seed of the best of your yearly crop. (I think) You'll be glad you did.