Here's a chart that will help.
https://thesquirrelboard.com/forums/...7&d=1232419601
The chart shows the calcium to phosphorus ratios of different foods. The reason corn is on the list is because of the bad Ca: P ratio. The best foods have a ratio of 2:1. Corn is a very high phosphorus food and in abundance would cause Metabolic Bone Disease. That's why it's limited. The spinach is limited because it is high in oxalates which can be associated with kidney stones. Kale is fantastic. It's a super food.
Sugar snap peas are fine. Squirrels love them.
The problem with bell peppers is actually anecdotal. We don't know for sure if they are bad. Several years ago there was an observation that squirrels that were vomiting had eaten bell pepper. It was decided to avoid them because of this. Because of the small population that this was based on we really are not sure about this. Avoiding them is basically out of an abundance of caution.