"List of chats for a single user" is available from August 25, 2021 Release Log.
You can use this Graph API endpoint to List all chats of users
GET https://graph.microsoft.com/v1.0/users/{user-id}/chats
To List messages in a chat you can use these Graph API endpoint
GET /me/chats/{chat-id}/messages
GET /users/{user-id | user-principal-name}/chats/{chat-id}/messages
GET /chats/{chat-id}/messages
You can track new features/changes in Graph API Using the below Documentation.
Hope this helps.
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