I, too have grown dried marrowfat peas from the supermarket, a 1kg bag. I used them for shoots (which I grew from time to time in a tray). Initially germination was good but did get worse over time and now after 4 years the last batch I tried was disappointing, but to be expected.
“they” say we shouldn’t plant supermarket garlic (and potatoes) due to the introduction of diseases. I understand plants grown for consumption aren’t checked the same way as plants grown for further cultivation. But it is expensive to do so; I was disappointed with my yield of official solent white from 2 bulbs bought for £10!!