Sorry, I didn't mean to suggest that it was a myth (bad wording on my part), just that I give mine perpetual spinach pretty much every day and don't experience problems of that nature.
Yes...Although the risks are high so I never recommend it when so many other great things to feed..The only thing I could find on that is that spinach contains another substance that reacts with calcium and ties it up in a form that the body can't absorb, so extremely little of the calcium in spinach is actually absorbed by the body. On the other hand about 50% of the calcium in kale is absorbed by the human body during digestion, which is pretty high. I don't know if this is the basis for that belief or not.
Mine get a lot of kale during the season, which is mid spring to mid-summer. I have not seen any effect on egg shells. I can't get spinach to grow here well enough to make it worth planting.