I thought the protocol being added to Swift 5.1 and its associated standard library. My application is being built with the iOS 13 SDK using Swift 5.1 and I can use other features like Property Wrappers from my swift packages. However, even though I can use Identifiable in my application target (built with the iOS 13 SDK, targeting iOS 10), I cannot use it in a package that declares .iOS(.v10) as its platforms (even though this is also being compiled with the Swift 5.1 compiler). It results in an error stating that Identifiable is only available in iOS 13 and above.
You can see that it's annotated as being iOS 13+ here too: