It depends on what you mean by toxic, but most healthy animal foods contain basically zero antinutrients or toxins. (There may be a small amount environmental toxins.) Thus animals whose diets consist almost exclusively of prey do not evolve livers that are good at filtering out toxins.

While I don't mean to imply that all plants are going to be like poisons to cats at any dose, almost all plants do contain antinutrients, which can be potentially toxic at some dose to an obligate carnivore.

Most of the toxin load in omnivorous diets, including the typical human diet, comes from stuff plants produce naturally. This makes a lot of evolutionary sense; chemical defenses are the only ones plants have and there is usually a subtle mix of nutrients and antinutrients even in popular domesticated foods. Apple seeds contains cyanide. Wheat contains lectins and phytates that block mineral absorption. In India's largest chickpea producing region they have to put a price floor on chickpeas, otherwise poor people will eat enough of them to die. I am not exaggerating when I say that basically all plant foods contain antinutrients, and like anything it's the doses that makes the poison.

(Another subtlety is there are few pure antinutrients. I mentioned phytates, which can cause mineral deficiencies in humans at high enough doses. But phytates may also be protective against cancer. This doesn't change the basic argument I'm making about whether cats are designed to filter out all the chemical warfare that plants unleash, in significant doses.)

This is basic stuff, although contrary to many people's uninformed opinion. Feeding a bunch of plants to a predator is dangerous in the short term and abusive over the long term. I'm sorry I don't have any specific links for cats and liver toxicity specifically, but in general you can look to "The Killer Salad", which is a chapter in one of Michael Pollan's books, or to the writings of Paul Jaminet if you want to verify what I am saying bout the majority of toxins in mixed diets coming from plant foods.

Here are a couple of cat specific links that only mention the liver/toxin issue in passing, they look at the issue more broadly:
https://animalwellnessmagazine.com/cats-cant-vegetarian/
http://messybeast.com/veggiecat.htm

If you google for example foods and plants that are considered safe for dogs to eat -- and then compare to cats -- you'll find the list of stuff that's safe for dogs is dramatically longer. Same thing for essential oils, many of them can be very damaging to cats in small doses. It's because of what I have explained above.. predator animals are not evolved to filter out stuff the way others are... they are designed to eat animals who have livers evolved to do the filtering for them. There are plants described as "safe" for cats to eat and you will occasionally see a cat eat small doses of random plants. They are talking about occasional consumption. But making any plant the staple of a cat's diet is cruel. Imposing ideology onto nature requires ignoring a lot of evidence.

The burden of proof should be on those who advocate feeding an animal something far different to its evolved diet. Still, there is lots of evidence that cats do poorly the more their diet gets away from not just animal foods but raw animal foods... I cited a well known study that's quite old above, but I'm absolutely sure you can find more evidence on this, if you care to.