Soybean seeds, as in this article, are not hybrids, they're inbred varieties. Same thing with wheat. Still mostly the same with rice, though there is hybrid rice, but it's not nearly as common as inbred rice.
Corn is a hybrid because hybrid vigor offers so much better performance and it's easy to detassle so you have female-only plants to be pollenated by neighboring plants.
I won't address other plants, but as far as I'm aware, corn is the only major crop that is a hybrid.
Plants that reproduce the same year after year are likely to be inbreds that is not equal to heirloom. I can think of plenty of rice varieties that are modern mega-varieties that have displaced older landraces, neither one is a hybrid.
In the US it's not common to save seeds year after year because the quality offered by seed companies is better. To effectively save and clean seed can be done, but the time and equipment required usually means it's more efficient to buy new seed every year. This differs from crop to crop and by country/region. But in this case, I disagree with the assumption that farmers "regularly" propagate, sow, harvest and save seeds (in the US).