It’s still there, but it’s not activated by default. Open up a terminal, then enter the following command:
dconf write /org/gnome/mutter/dynamic-workspaces true
Now, when fireing up the workspace/task-switcher, you’ll be able to create additional workspaces:
Please note that it’s been disabled by default for a good reason - it has some quirks with Endless’s desktop environment. Just check if you can life with them - if not, you can revert to the original state by issuing the following command:
dconf write /org/gnome/mutter/dynamic-workspaces false