This is pretty common in 3D work. Blender has a feature called “blend shapes” that implements a similar interface, and is commonly used for complex facial animation and general model parameterization.
That's cool - I had been thinking with the OP site that quite a number looked like they were scowling/angry so a way to vary that is interesting to explore.
He has a bunch of other interesting things like this, too: https://www.redblobgames.com/