Ctrl+Alt+Arrow (switch workspace) won't work even if workspaces are enabled. Instead, it seems to have other effects in several programs

You can resolve this issue by using dconf-editor. It's available by first enabling the universe repository and then issuing the command sudo apt install dconf-editor After which you can launch it from the applications menu. See below for the relevant section:

Navigate in the left hand column down to wm, click the arrow that points to the right to expose and then select keybindings. Scroll down the right window to expose "move-to-workspace-down", select it and click the Set to Default button. This should set this binding to ['<Shift><Control><Alt>Down'] Repeat this process for the next 3 entries which correspond to left, right, and up. The changes should take effect immediately. If not, reboot.

kudos to @doug for this comment which pointed out the right place to look.

This solution restored the functionality, although it threw the baby out with the bathwater and didn't help to identify the cause:

If you want a hatchet approach, open ~.config/dconf and rename user to user.bak

Then immediately go CtrlAltF3, login, run the reboot command.

— doug