It's sad I have to even say this. Believe it or not I have already tried finding an answer to this question.
How do you close a workspace in GNOME(specifically, Ubuntu 18.04)?
Workspaces are automatically closed when you remove all the windows from it and switch to another one. I'm not entirely sure, but I think on Ubuntu 18.04 it might also keep one open at the bottom so you can drag windows into it.
Also see: https://askubuntu.com/questions/1047051/remove-a-gnome-workspace
It seems that
Ctrl+Alt+left/right arrow can switch the workspaceCtrl+Alt+Shift+left/right arrow can bring your current window to the target workspace(create a new one if not exists)
on Gnome ClassicIn addition to that the size of workspace is configurable. Which also defines the behavior whether a workspace will be closed while no using.