You asked to include why so I'll comment on a few things.
Apple seeds - Many fruit seeds contain arsenic. It's here to kill any tiny insect that might try to eat the seed. Have you ever swallowed an apple seed? I did when I was a kid picking apples off of a tree. Why am I not dead? Dosage. The amount of arsenic in an apple seed might kill a tiny insect but isn't enough to bother me. It's not enough to bother a chicken either. I have fruit trees either growing in the run or nearby. They eat fruit that falls or I toss fruit into the run. That fruit has seeds in it but they don't eat enough seeds to harm themselves, not by the time they fill up on the fruit. I've found plum pits in their gizzard when I butcher them. Those have arsenic too. It does not bother them. When I make apple sauce or apple jelly I don't give them a pile of pure apple seeds to go through, why take a chance, but an occasional apple seed when eating an apple is not a problem at all.
Avocado skin and pits - I can't remember exactly what part it is but there is a part of an avocado that can be harmful to them so I avoid them getting any avocado.
Dry beans - Uncooked dry beans contain a substance that can be harmful to you or chickens. Different varieties of dried beans contain different concentrations, red kidney are the worse. Cooked dried beans are OK for you and them. Dosage comes into play some but just avoid all uncooked dried beans.
Dry rice - It would be interesting to know why people think this is bad. There used to be a myth that throwing rice at weddings was bad because wild birds would eat it, their crops would swell up when the digestive juices hit it, and that would kill them. As I said, that is a myth. Chickens and other birds eat all kinds of dried grass seeds (also called grains) and don't have problems. I don't hesitate to feed dry or cooked rice to mine. By the way, it is still a bad idea to throw rice at a wedding. Not because of any danger to the birds but on hard floors or sidewalks it is really easy to slip on those dried rice grains and fall. You could hurt somebody from the fall.
Raw egg - Again, why? Chickens don't know what the inside of an egg tastes like until it is opened. How can eating raw egg cause them to open an egg eat it? It can't. It is pretty common for a chicken to eat an egg that is broken, that's just taking advantage of available nutrition. That doesn't make them an egg eater. An egg eater is a chicken that learns to open an egg to eat it. Those are really pretty rare. I've had one in my life. When she opened an egg other hens would help her eat it but none of the others ever learned to open one on their own. I prefer to cook eggs before I feed them back to the flock, but that's mainly because of the mess raw eggs can make. They are not like a dog that could lick it all up, they can leave some to go rotten. I hate that rotten egg smell.
Raw potatoes - another one where myths abound. There is nothing wrong with you or them eating raw or cooked potatoes. The problem comes in when those potatoes are green. If a potato is exposed to sunlight it turns green. The green itself isn't the danger, but a potato in sunlight also produces a toxin that can harm you or the chickens so the green is a sign that the toxin is there. Cooking doesn't help, cooking doesn't destroy that toxin. Dosage again comes into play, one bite won't kill you or them, but I try pretty hard to not feed my family or my chickens any green potatoes damaged by exposure to sunlight.
Salt - If you look on the label on a bag of chicken feed you'll see salt listed as an ingredient. Some salt is good for them, very small amounts of salt. But, a big but, too much salt is not good for them. Sea birds like gulls that drink salt water can separate out the salt. Chickens can't do that. Again, one bite won't kill them but avoid salty foods. They'll pickup what they need without you feeding it to them. To be clear, avoid salty foods.
Mine get table scraps, kitchen scraps, excess from the garden, whatever they can find when they forage, including a fair amount of fruit and berries in season. You can see from the above what I try to avoid. To me the key is moderation. In general, one bite won't kill them, let alone harm them. So moderation goes a long way to keep them safe. By feeding all treats in moderation so most of what they eat is their chicken feed you also help them keep a balanced diet which protects their health.
