You don't really have to go far back for plant foods we look at as essential today to not really exist, for several reasons. Importing of things out of season is fairly new. Most of those things are never in season where you're at. Some oranges for Christmas when my parents grew up. All extremely hybridized to be sweeter and larger.
Veggies, too. Lethal until we breed most of the worst toxins out og them. Some are still really bad. Arugula is not too bad. Kale and spinach though really is. Cooking helps, but what does that say? Yes, we cook meat, but it's not for toxins or digestion, it's just smart when dealing with wild meat to kill parasites.
Traditional plant food? Akorns that you rinsed for days or weeks, other things were fermented, then dried and you made some sort of bread out of them. That's plant food. Grains are pretty new as well. Easy to grow survival food, but when that became the norm is when our teeth got shot. Grinding on rocks didn't help either, but that's more physical wear and tear than the por nutrition it already represents.
Here it would be 3-5 week long season of various berries. Maybe some kelp. Spices? Well, I think Norwegians adopting anything more than the typical bread and cake spices happened pretty much during my lifetime, so..
Prior to potatoes the vikings had angelica, broad beans, carrots, onions and garlic. Those onions and carrots still were not what we would recognize today and that's still pretty recent. The Vikings series didn't get much right, but snacking on dried herring was probably accurate. I'm growing angelica myself now just because.
Plants are medicine for other carnivores as well, but you don't take medicine every day. If there's an issue you take it as long as the benefit is greater than the harm it does.
It's quite possible that some populations are well adapted to some plant foods if that was an option they were dependent on. White rice would be one of the more obvious ones. Stored better, yes, but you also got rid of the fiber and other anti-nutrients.
There's all sorts of stuff. The vegetarian thing is yoga seems to be far from it's origins. Fish and bread in the Bible probably meant meat and fish.
That any green vegetables are somehow essential for human health? There's absolutely no real evidence. Evidence of harmful effects though plenty. Goitrogens. A real issue.
Thee's a case to be made for vitamin C, but then again, if you don't eat the carbs it's not an issue. If you understand what processes require vitamin C that's clear. If scurvy was a concern we'd see a lot of that in carnivores a long time ago. Obviously, an issue if all you eat is hard tack and salted herring, but then we know why it's an issue.
RDA values make little sense, Mr. Keys was behind those as well btw., as those change very much depending on what your diet actually is. With animal foods everything you need is provided in it's most bio available for. If you stick to ruminants, that's even more true as issues with pork, chicken etc are quite common. I also generally eat only the yolk of eggs, as the whites do nothing for me, just as a wild fox would do.
I still have a few plant things I've not lost my taste for and I don't really think they're anything special when it comes to nutrition. They're simply not the worst and I happen to enjoy them occasionally. Avocados and potatoes are at the top with potassium, at least if were talking about something that's easy to eat in relevant amounts. A pound of kale really isn't.