I am currently stuck on macOS 10.13 and therefore need to keep using Xcode 10.1. The Swift 5 final release from Swift.org - Download Swift was compatible with that Xcode version. However, with the Swift 5.1 final release, I am getting an error <unknown>:0: error: Swift does not support the SDK 'MacOSX10.14.sdk' when building. In Unable to build: "Swift does not support the SDK 'MacOSX10.14.sdk'" @jrose indicates that the Swift 5.1 snapshots required the Xcode 11 beta; does this apply to the final version as well?
Yes, nothing has changed there. The tying of a branch to a particular SDK is something we Apple folks might be able to change in the future, but for now the requirement is still that each branch requires the SDKs from the Xcode it is associated with.