I have watched a LOT of gardening videos and many tell you the same about what won't break down. THAT isn't true. Egg shells Will break down, but it doesn't happen fast. I saw a gardening program featuring High Mowing Seed Company. Owner, who lives near the Atlantic Ocean, turns in clam shells in his acreage. One time a county Extension Officer examined his soil and thought it foolish to do so. HIS response was, that, no matter how long it takes to break down, it Will and the soil will be better for it.
This is a very loose paraphrase of his answer.
MY feeling is that anything that we can eat, or the natural packaging, like eggshells, belong back in the soil and Not in a landfill.
I Don't have the time to constantly break down eggshells, so, right now, they are going into my composter for eventual garden bed filler.