There are still rarer cases where callback refs are useful. For example when you need to run a side effect on a node as it attaches and detaches.
Also note: legacy lifecycles are not deprecated yet. You won’t see the warnings unless you opt in with <StrictMode>. They will be officially deprecated in a future minor release when more libraries have had time to update.
You asked "Curious, what do you use it for?" in regard to componentWillReceiveProps
https://news.ycombinator.com/item?id=16695064
I use componentWillReceiveProps for just about everything - I had thought it was a fundamental part of the Redux flow. i.e. Redux updates, new props flow through the hierarchy, I pick that up in componentWillReceiveProps and make changes based on new props.
Is this not correct?
What you're describing sounds more like you're updating something external (e.g. your Redux store). This isn't what the lifecycle is meant for, and you'd be better off using componentDidUpdate instead.