Currently, Xcode and Cocoa Touch fully support app development in both Swift and Objective C (either or both). So you can use either for iOS apps for which you can find decent API documentation. However, a lot of Apple's new documentation is Swift only.
Except for the evil stuff that Swift requires to get access to C pointers and memory, it's pretty easy for an experienced Objective C programmer to learn Swift 3 or 4. A few weeks of study to fall on all the gotchas, and a couple learning apps in Swift should do it.
So my earlier apps in Objective C, I'm keeping them in Objective C, even when doing significant updates. However any new apps that use completely new APIs, such as ARKit, etc., I'm writing in Swift.