What the docs actually recommend (although it’s badly worded) is that you should treat key and ref as internal to React and not accessible inside the component. If you need to know the key, simply pass it as another property with a different name and then access it on this.props as normal.
Quote from above:
You can no longer access this.props.ref and this.props.key from inside the Component instance itself. So you need to use a different name for those props.
An example would be:
<MyComponent key={foo} reactKey={foo} />
Then accessing inside as this.props.reactKey.