I'm trying to understand why this isn't a thing already. It seems there would be a market for it; when you consider all the different keyboards shapes and sizes...

A UK company had produced them for decades, which probably serves most injured non-geek users.

https://www.maltron.com/store/p19/Maltron_Single_Hand_Keyboa...

Check out chorded keyboards. They've been a thing for a very long time. At least since early 00s or 90s when I saw them first. They are held one handed have 5 keys and you get different letters by chording multiple keys together.

You can just buy a split keyboard and put all the keys on layers on one side.

There's Maltron, Microwriter (who pretty much invented the contemporary chording ASCII computer keyboard) and its weird successors like Twiddler and Charachorder.

But the fundamental problem with one-handed keyboards is that as soon as you only have one hand, you step into specialisation.

People's hands and one-hand abilities are actually quite variable. People who have never had two hands have different hand agility to people who lose a hand in adulthood, for example.

Two-handed keyboards and two-handed typing masks so much of this variability, because you can be a fast and efficient typist even with your hands straying across the keyboard and using only two or three fingers on each hand (say, two on non-dominant hand, two and thumb on dominant).

One-handed keyboards, by contrast, need to be more optimised for individual one-handed typists when any economy of scale is already difficult to achieve.