This has now moved from PC Settings..General to Devices..Typing (start by swiping in from the right and selecting AllSettings)
It's so thoughtful of MS to have it turned off by default, wouldn't want people to accidentally find out it was available after all, far better to hide it in a setting screen somewhere and make people waste their time figuring it out.
Of course the only reason I needed to figure this out is because Chrome has suddenly decided that alt <- is a better back key than backspace.
Gotta love technology.