Mayo is basically eggs and oil.
We Jews have all kinds of crazy laws about what is and isn't kosher for Passover. Like, "if the Hebrews didn't have it during the exodus from Egypt, it's not kosher."
Well, they had chickens, so therefore they had eggs, and they had oil, because the Bible mentions oil.
So therefore, mayo is kosher for Passover.
But they sure as hell didn't have a ham sandwich to put it on.
Must have been Chicken Salad. ;~))
Well, mayo is eggs, oil, and an acid such as lemon juice or vinegar. But the migrating masses didn't have a plethora of modern foods either.
I'm losing my appetite...