There are a lot of different opinions on what and what not to feed them. Citrus is a red flag to some people on here. Some people say to stay away from it. Some people mention a study in Florida where it was fed to chickens and they were fine, but I can't remember if that was meat chickens, a laying flock, or something else. And I don't know what percentage of their overall food was citrus. I have not heard what in citrus is supposed to do to harm them.
Then you get a lot of people confused. The green potato skins contain a substance that is harmful to chickens and people. The green potato skins come from the potaotes being exposed to the sun. Regular potato skins are fine for chickens and us. But some people don't read the green in that and are adamant that all potato skins are bad for chickens. Nope. Just the green ones. I suspect that is what is happening with the tomatoes. I feed my chickens tomatoes all the time in the summer. It is not a problem. I have read that the tomato plant itself is bad for them, not the tomato fruit. The tomato plant is a member of the nightshade family, but the fruit is fine for them.
Then you have things like beans. Uncooked or undercooked beans contain a substance that is bad for them. Fully cooked beans are fine. So whether they are cooked or not can enter into it. The temperature you cook the beans changes that substance so they are OK, but the green potato skins do not change with normal cooking temperatures.
Then you have the issue of how much. Chickens are not normally going to drop dead at the first bite of something that is not good fo them. It takes a certain amount of "poisin" to do the job. I'll use apple seeds as an example. Like many fruit seeds, apple seeds contain cyanide. Cyanide is a deadly poison. Have you ever swallowed an apple seed? Then you swalowed cyanide. I have, yet I live. An apple seed does not contain enough cyanide to harm you or a chicken. Your chickens would have to eat a lot of apple seeds before they are in any danger. I have absolutely no problem with chickens eating apples and the apple seeds in my orchard. They won't get enough to hurt themselves. I throw apple cores with seeds on my compost pile and the chickens get them. But when I make apple jelly or apple sauce, I don't give them a huge pile of almost pure apple seeds. Would I kill my chickens by dumping a huge pile of apple seeds where they could get to them? Probably not ,but since I know apple seeds may be harmful to them, I take what I consider reasonable precautions.
Same thing is true of green potato skins or uncooked beans. One bite won't kill them. When I pull the old bean vines from my garden they still have some uncooked beans attached. I don't worry about those. The chickens won't get enough to harm themselves. But when I pick and sort my dried beans, I get a lot of rejects. I don't dump a huge pile of uncooked rejects where the chickens can get to them. When I sort a few dried beans just before I cook them and get a few rejects, these wind up on the compose pile where the chickens can find hem. Reasonable precautions, but don't obsess over it.
If you keep them contained and you provide all they eat, they need a balanced diet. Chicken feed provides that and should be the majority of what they eat. The rule of thumb is to only provide as treats what they can clean up in 15 to 20 minutes. Like most rules of thumb, this is very safe.
As far as I am concerned, it is all things in moderation. If you consistently feed the same thing as a treat, don't feed a lot of it. If you vary the things used as treats, then you can feed more. And by varying it, I don't mean cabbage one day and kale the next. Those are essentially the same thing. I mean varying between tomatoes, peppers, squash, cucumbers,corn, bugs or worms, green stuff and whatever else I have.
So will tomatoes or citrus harm them? In moderation, I sincerely doubt it. In excess,possibly, but so will practically anything else.