The classic workspace no longer ships with AutoCAD. However, you can easily recreate Classic workspace with following instructions:
Click Quick Access Toolbar drop-down > Show Menu Bar for displaying the menu
Hide the Ribbon via Tools > Palettes > Ribbon.
Select the toolbars you want to display via Tools -> Toolbars -> AutoCAD
Repeat until all required toolbars are visible.
Save the workspace via Tools > Workspaces > Save Current As. Name the new workspace as AutoCAD Classic.