I've had it (szczaw in Polish) a few times
The same. We have it in Britain too, Sorrel, also used for soups, but it's not at all common. If people have it, they've grown it; it's not in the shops.
Sometimes at home in Warsaw if it's the other cooking rather than me we have sorrel soup though I'm not a big fan. As Paw said, there are problems. You really do have to be careful about having too much. It's high in Oxalic Acid (like rhubarb leaves) however some (Ukrainians anyway) say that putting sour cream in it mitigates that, I'm not convinced.
My dentist once said that dried szczaw in soups had historically kept a lot of poor families in the east alive over the winter but had also shortened their lives through overuse.
(turnip greens, mustard greens and collard greens are all eaten on a regular basis in the US South).
I'd like to try those. Mustard leaves exist in the U.K. because they're used in curries. Indian and Pakistani families used to plant them in the garden since they didn't exist in the shops until recently (and you'd only expect find them in a town with a significant south Asian population). There's something there called Spring Greens that I like but which are a bit old fashioned, sadly.
The leaves of young beetroot (as close as damn it to Swiss Chard) are a different thing. Botwinka is, in my opinion, the best of Polish soups and I make it often when the beetroots are young. I prefer it without buttermilk which swamps the delicate taste and the nice muddy dark red colour, however ours is a mixed household and Ukrainians always put buttermilk in it.