The fantastic Can I Use website has some great information on which browsers and versions support Intl.NumbeFormat:formatToParts(), however their data around Safari largely says "Support Unknown." I needed to know specifics, so I did my own test using BrowserStack.

It seems on both Desktop and iOS that Safari 13 has support for formatToParts(). Previous versions do not.

That means that iOS 13 and later are the only versions of iOS that come with Safari 13. All previous versions of iOS won't support this feature. Pretty lame, Apple!

Note: iOS 13 is the latest version of iOS as of writing.