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.