It doesn’t make sense to me – they’ve always gone in the trash.
I’ve taken a class in water treatment but eggshells never came up. I’m not sure how far they make it down the sewage. I assume they sink and become part of the sediment.
Regardless of where they go, eggshells in the garbage disposal is a small annoyance. To me.
It is however, unlikely they’re causing any problems.
I can get so caught up on style – mow the lawn around the edges first or leave those adjectives out of that sentence – that I ignore the results. And usually the results are equally good.