Hi Mary,
My name is Didi, an Independent Advisor. I'll be glad to assist you today.
Adding or including the chat history applies to a group chat, not one-on-one.
Teams will save the whole chat history, all the way back to the first message. And if someone leaves the group, their chat responses are still there in your chat history.
More information on these links:
https://support.microsoft.com/en-us/office/star...
https://support.microsoft.com/en-us/office/six-...
Thanks!