It seems like almost always if you want to do anything during unmount, it's cleaning up something you set up in a useEffect call, which you also want to do any

It seems like almost always if you want to do anything during unmount, it's cleaning up something you set up in a useEffect call, which you also want to do any time the useEffect call's dependency list changes, so it being part of the useEffect hook helps programmers fall into the pit of success. React's older class-based components had the unmount handling separate (componentWillUnmount), and in my experience on a large React codebase, many if not most components using componentWillUnmount were subtly flawed and failed to handle certain cases where props or state updated in a way that should have caused effects to be cleaned up or adjusted. React's useEffect hook way of grouping up effects with their own cleanup code helps people handle these cases correctly even without realizing it sometimes.