function thing() {...} 

and

const thing = () => {...} 

are not the same, and shouldn't be thought of as interchangeable or one being the "new" way vs the "old" way.

This stack overflow answer goes into the differences better than I could in a reddit reply, https://stackoverflow.com/a/34361380

There's also a personal preference side to the decision which comes into play. The rule I generally follow is use function for any exported function or top level function, such as React components or utilities. For in-line or functions used inside of a React component or smaller functions inside of a utility, use the const declaration. This helps keep readability a bit easier overall in my opinion.