'Switch to last workspace' and 'Move window to last workspace' shortcuts are only useful for dynamic workspaces
These shortcuts are useful for dynamic workspaces as the last workspace is (usually) the next empty workspace. When using fixed workspaces it is rarely (if ever) useful to switch/move to the last workspace, but it is useful to be able to switch/move to the next empty workspace.
This could be done without changing the behavior of these shortcuts for dynamic workspaces, but the shortcuts would need to be renamed for clarity.