While we have added and modified some features, we have removed the Custom Lists feature in Documents. Here is why:
V8 and prior versions had three ways to organize documents - Tags, Lists, and Folders. Since Lists and Folders were similar, they created confusion and UI complexity. Hence we have removed the Lists feature in documents to simplify the process.
Now in V9, you can continue to organize documents using folders and tags (private and shared tags)
To know more about What's New in V9, click here