I think/use "on the list" usually but in python programming I tend to think "in the list" when dealing with lists. Probably because a list in a computer program you often do not have a snap shot of it in your mind. The contents are often ambiguous. You have the "in" operator in python to test if an item is in/on a list too.
When I hear "on the list," I imagine a conversation like this. "Where is the grocery list?" "I put it under the keys so you won't forget it." "Oh, so the keys are ON the list."
You aren't buying new keys as part of the grocery list. You buy the things listed IN the list.
Native speaker, and I asked around. Everyone I talked to IN Indiana (not ON Indiana) said IN the list.