It still works in Big Sur! Just hover over the window title and the icon (called the "document proxy icon") will appear.
I think it's fine that they hid it, since even before, the things you can do with it (dragging, cmd/ctrl-clicking) were already hidden.
The only old functionality that is lost with Big Sur is that you could glance at the icon to see if there are unsaved changes (if so, the icon would be dimmed). But you can still tell by looking for the dot in the red close button.
I had no idea one could do this!
But then there are core features like "a file icon on a document window can be dragged exactly like a file icon in the finder" that you can use macOS for years and never find. That's before they started hiding the file icons in Big Sur!
macOS is packed with little things like that.