It's first worth distinguishing "the Swift runtime and standard library" from "the SDK". Combine, SwiftUI, CryptoKit, and RealityKit are frameworks written in Swift, but they're not part of the Swift Open Source project. They're more like ARKit, MapKit, and Intents: part of Apple OSs, made available for use by apps via Xcode and its SDKs.
Stepping beyond that, an SDK doesn't contain enough information to run things; only to build them. There are many reasons for that, but fundamentally it comes down to some changes having to happen in the operating system itself—either because the changes actually need to be in the kernel, or because something OS-internal has changed in a non-backwards-compatible way.
So Xcode 11 on macOS Mojave can't run apps that use Catalina features like Combine. However, what is part of Xcode is the simulator, and so you can write apps for iOS 13 and try them out in the simulator. And playgrounds are built on top of the simulator, so you can experiment there too (as long as the playground is building for iOS or tvOS and not macOS).