There was a brief period where they seemed to be insisting on Objective-C, but it didn't last long. Now they don't care what language you use as long as your functionality follows their guidelines.
This was brought up a couple days ago during discussion of a game built in Rust on the app store. A comment there[0] claims that the restriction on languages is no longer in the guidelines.
In fact, it was only in the guidelines for a few months back in 2010. I can only quote the response to the comment you linked:
> This rule got so much press 7 years ago that it will be one of those "truths" about a platform that never dies on internet forums, that and the Playstation platforms using OpenGL as their primary API...
Google has had a Go app, "Ivy," on the app store since 2015. I think it was just an experiment at getting Go to build for the App Store and getting it approved, but it was in fact approved.
It's true that fork() isn't allowed on non-jailbroken phones, so you can't shell out to a Go binary (you can on Android if you want). But you can build a static library in Go and link it and call into it with cgo from Objective-C or something.
