I feed mine greens from the garden, spinach, kale, chard, lettuce, cabbage, turnip, and beet tops. The chickens are extremely healthy and lay nice hard-shelled eggs. They also get a lot of vegies like tomatoes, ripe peppers, squash split open, corn scraps when I can corn, just about anything that is not suitable for the table or when I have excess. I pretty much toss it into the run and let then decide what they want to eat. When I can beets I toss them the cooked skins. If you do that don't think what you see in the coop the next day is bloody poop, it's not. Red cabbage causes a sort of blue poop. When you feed them stuff you can get surprises.
As far as kitchen scraps I toss most things to them and let them decide what they want to eat. They usually like it better cooked than raw but will eat a lot of things raw. I don't worry if it has butter or herbs and spices in it. The one exception is if something is really salty, they don't handle excess amounts of salt well. The key is moderation, don't overdo any one thing. In general one bite won't kill them, they'd have to eat a pretty large amount of most things before they got a harmful dosage.
I avoid feeding them avocado, even small amounts can be dangerous. Uncooked dried beans pose a risk, they do to you also, but if they are well cooked they are fine. Do not feed them or eat your self any potatoes that have green skin. The green-skinned potatoes contain a substance harmful to you and them. Regular potatoes and potato skins are fine, mine prefer them cooked but will eat them raw. It's the green ones that are dangerous.
Practically anything you eat or they eat contains something that can cause harm if consumed in huge quantities. You can find warnings on the internet about practically anything. Cabbage is a great treat, cooked or raw. But if you feed them nothing but cabbage over a time period it can cause thyroid problems. Cabbage would have to be a huge part of their diet to pose a risk. Most fruit seeds contain a small amount of poison, you'll often see warnings about that on here. But there is not enough poison to cause any harm at all to chickens in a few seeds. The dosage in an apple seed is meant to kill a tiny insect that might bore into an apple seed, not something the size of a chicken. My chickens forage in my orchard all the time and eat a lot of the fruit that falls off the trees, seeds and all. When I can apple jelly or apple butter I don't leave a big pile of the seeds where they can get to them, but it doesn't harm them at all if they eat a few seeds. Moderation.
I grew up on a farm in the ridges of Appalachia. My family had lived on similar farms for many generations and all kept chickens. From spring when we got them until fall when we butchered all kitchen scraps went to the pigs. After hog butcher time, the kitchen scraps were tossed where the chickens could get to them. They ate what they wanted and left the rest. They did well on that, especially over winter when foraging was a bit rough.
Some people like to micromanage every bite their chickens eat. They can get really obsessive about it. I'm a lot more laid back about it. As long as things are fed in moderation I'm not worried though I do avoid a few certain things.
If you keep them confined so they don't forage and are dependent on you for everything they eat, their commercial chicken feed contains a balanced diet, everything they need. The general recommendation is that treats (anything other than their regular chicken feed) should be less than 10% of their overall diet. That way they get all the nutrients they need and maintain a balanced diet. Moderation.
Of course this is my opinion based on my experiences. You will see that others have different opinions. The hard part of the internet is that you can get so many totally different opinions you have to decide who you believe. Good luck!