https://www.backyardchickens.com/a/chicken-treat-chart-the-best-treats-for-backyard-chickens
That's a link to the Treats Chart in the Learning Center on this Forum. If you look at the bottom of the chart, you will see some things that you are not supposed to feed your chickens.
You will find that several people disagree with this chart, some pretty strongly. There are a few things you probably should be aware of.
It helps to be able to read. You will notice that it does not say that potato skins are bad for your chickens. It says that green potato skins are bad for your chickens. There is a difference. It does not say that all beans are bad for them. It says raw or undercooked beans are bad for them.
Usually, one bite won't kill you. Your chickens are not likely to drop dead the instant they take one bite of something they should not. I'll use apple seeds as an example. Apple seeds contain cyanide. Have you ever swallowed and apple seed? I did several times when I was a kid on the farm. Yet I'm not dead. Why? you might ask. Many toxins require certain levels to cause damage or death. There is not enough cyanide in one apple seed to harm me or my chickens. I don't worry a bit if my chickens eat apples, including seeds, in my orchard. They are simply not going to eat enough to cause harm. When I make a lot of apple jelly or apple sauce, I have a lot of seeds left over. I'm talking about several quarts of sauce at a time. I don't dump that pile of concentrated seeds where the chickens can get to them. Would they eat enough seeds to harm themselves if they found a large pile of concentrated seeds? Maybe, maybe not. I'd even say probably not. But since I am aware that a lot of apple seeds might be harmful, I take what I consider reasonable precautions.
At the same time, realize that not all damage to them is immediately apparent. Just because they don't fall over dead the instant they take a bite of something doesn't mean it is not causing damage to their internal organs. Usually it takes really high concentrations of something to harm them or they need to eat it for a period of time for the cumulative effects to build up and cause harm. An easy example of this is feeding chicks Layer with the extra calcium in it. There are plenty of studies that show that the extra calcium in Layer can cause damage to growing chicks. Can, not necessarily will. One bite of Layer will not kill them. There is not enough in it to cause them harm from one bite. If they eat enough Layer over time for the cumulative effects to build up, the excess calcium can damage their internal organs to the point it weakens them enough that they die or are not as hardy as they should be. They may die a year later from stress that they should have been able to handle. Plus, if they are eating a lot of other things besides the Layer, maybe they are not really getting enough extra calcium to harm themselves by eating a bite or two of the Layer. What is important is the total amount of calcium they eat in a day, not just in one bite.
Many of the things on the “good” list contain things that can harm them. Cabbage for example. If they eat a tremendous amount of cabbage, they might eat enough to cause themselves a problem. But it requires a tremendous dosage for any harm to occur. There is a real good chance they will fill up and stop eating the cabbage before they eat enough to cause any problems, but don’t feed them huge amounts of cabbage or anything else consistently day after day. They need a balanced diet, not one concentrated in one or two things. If they are able to forage for themselves, they will take care of a lot of this themselves. But if you provide all they eat, just give them a variety of treats, not always the same thing, and rely on their regular chicken feed to be a major part of their diet. Moderation is the key.