I know modern vegetables have been modified over generations for various reasons, that leaves the question of that if I went into a medieval European vegetable garden what would I see? How different would modern vegetables look? Would they have extinct vegetables/varieties?

A medieval garden would absolutely contain some plants that are not familiar to the modern market-goer or even vegetable gardener, and some that are there would look weirdly different.

First, and probably foremost, you would see no potatoes, tomatoes, bell or chili peppers, pumpkins, or sweetcorn. All of those come from the New World, and weren't in Europe in any meaningful way until the late 1500s.

Brassicas - of which we have cabbage, brussels sprouts, cauliflower, broccoli, kale, and some others - would not be the same. Brassicas vary wildly, in that the above are essentially just cultivars of the same species, brassica oleracea. In its wild form, it looks vaguely like kale. Occam's Razor suggests that that's what you'd see in the medieval garden, too, under the name of worts, but in reality, there were probably dozens or hundreds of local variations on it, because as soon as you start using seeds from brassica plants you've grown, local adaptation happens quickly. So we don't really know what worts looked like, beyond "probably green and leafy". My best guess is that the one currently known as Good King Henry is a reasonable estimate.

Carrots and parsnips were smaller and narrower, and there was an additional vegetable like them, skirrits. Skirrits resisted the selective breeding that made the other two larger and wider, and remained as a mass of long thin roots, which are an absolute pain to peel, so they're nearly extinct now. Salsify was also more widely grown - it's a very hipster food at the moment.

Turnips would have been grown at a farm level, rather than in a kitchen garden, and would have been a staple foodstuff for many people. Beans (broad or fava), several varieties of peas, and similar pulses were field crops too.

Radishes and beetroot (beets) of various kinds probably haven't changed a whole lot. Lettuce and other green-leaf things (rocket, aka argula, and so on) would have been grown, but might not have resembled their modern forms too closely. They'd also have been more bitter, which is a trend throughout the vegetables of the medieval period. Rue, a plant now not much used because a lot of people are very allergic to it, was eaten in medieval Middle Eastern food a lot, and it too is bitter. Aubergines (aka eggplant) were also present, but smaller, and also more bitter, so much so that they needed to be extensively salted to draw out the bad-tasting fluids before they could be cooked. Indeed, that remained broadly true up to the early 20th century, so it's only modern aubergines that don't need that step.

Leeks, onions and garlic are probably almost unchanged. Particularly in the west and north of Europe, they were widely grown and widely eaten, and probably saved the diets in those places from being unbearably bland otherwise. Celery, while not saving anyone from blandness, was also pretty widely known.

Cucumbers, some forms of melon and gourd or squash, and courgettes (aka zucchini) would have been grown in southern Europe, although I think most if not all of them would have been smaller than modern varieties. Many would also not have been as sweet.

And finally, there're Alexanders and Angelica, two vegetables which were widely grown in the medieval era, and are now mostly weeds. They're tall, stalk-y plants, a little like branching celery, with white umbrel-flowers. Alexanders was boiled or fried, and while I've tried it, I can't recommend it; it basically tastes like a cut hedgerow smells. Angelica is still used in a candied form for cake decoration; it's bright green and has a kind of musky taste. I can't say I'm keen on it either.

And finally, there are a few herbs like lovage, savoury, and hyssop which saw a lot more use in the medieval era than now. Savoury in particular is really good, and I've never been clear on why it dropped out of use so much.