The following document states that some APIs in Storekit1 have been deprecated and that migration to Storekit2 is recommended, but I understand that Storekit2 only supports Swift. I'm currently creating an iPhone app in Objective-C, but is there a way to implement APIs equivalent to Storekit2 using a different Objective-C API?
The deprecated APIs still work so you could just continue to use them for the time being.
Otherwise, you will need to learn enough Swift to implement this part of your app using it. Combining the two languages in one app is not hugely difficult. How complex it is will depend on how much StoreKit functionality you need.