Shake to undo was actually a joke someone made in a meeting that became an actual feature.
I am so frustrated by every drawing app that doesn't have the same undo system as procreate. I don't know if it would work system wide (though I suspect it might) but for drawing apps alone it is an ergonomicaly wonderful convention.
Procreate's undo feature is great, isn't it? I've often tried to use Adobe's iPad apps after using it and Procreate really spoiled me because of its ease of use while drawing.