If it goes outside at all ... it doesn't.

We had a cat that didn't eat cat food from May until October, we didn't assume it was a breatharian during that time.

When our cat disappeared (we assume a fox or wolf got her) after almost a decade of this, the farmers to either side of us started having mice and rat problems again :-)

I'm not sure actually. In many videos, he offers it things like fish and chicken and wet cat food but that cat doesn't touch them. It does eat some dry kibbles - maybe that's enough? Now that I think about it, calling it "pure vegetarian" was my mistake because it does eat those kibbles which probably contain non-vegetarian nutrients.

Probably eats mice/birds/etc. or the food of other cats when the owner isn't looking.

I've been told that the cats of a great-grandparent were only fed pastas...

I've always wondered how they could survive on that! But I suspect that since those cats lived mostly outside, they probably ate many other things, including small animals they caught in gardens, sheds, etc.

And it's not just that cats have to eat meat to get nutrients they need. Their systems aren't able to process significant amounts of carbohydrates at all. A cat consuming more than a couple grams of carbs per day will develop kidney disease extremely quickly.

It seems plausible that those nutrients might be found in certain plants, and could be identified and concentrated. I wouldn't risk it with my cats, but plenty of people do feed their cats veggie kibble and I haven't heard of them dying of malnutrition in droves.