Swift officially supports Linux, and there is unofficial Windows support (and I remember them saying they planned to support Windows officially, though I don't know if that's still a goal).
The Swift support on Linux is the bare minimum required to say "Swift supports Linux". I seriously doubt that Swift will ever be any more "portable" than Objective-C. That is, Swift, like Objective-C, is portable in theory, but nobody uses it for anything except to write programs for Apple's iOS/macOS platform.
Seems to me like the Server APIs group (https://swift.org/server-apis/) is pretty much about running Swift on non-iOS/macOS platforms.
Ibm has extensive support for swift on linux, and seem to be pushing it as a first class solutikn for thier linux based develooment.