I've figured out you can turn shadows on/off by toggling the "Advanced Materials" button, but it may come in handy to have a shadows on/off master switch.
I also searched the Lua commands and the only shadow command I could find was at the entity level. On published games, when giving the user the ability to customise the graphics settings for their systems I'd think being able to turn shadows on/off for system performance reasons will be highly desired.