Ubuntu is supported (see https://swift.org/download/), but Windows is pretty early I think.

See https://github.com/apple/swift/blob/master/docs/Windows.md and https://forums.swift.org/t/windows-nightlies/19174 for more info.

Ubuntu is "supported". The compiler might be available, but there are hardly many libraries available that would compile outside Apple platforms.

That's no longer true. The Foundation framework is basically complete on Linux, and the vast majority 3rd party libraries which are not iOS specific will work on Ubuntu. Even many of Apple's own libraries (i.e. SwiftNIO, a low-level, high performance networking library for things like implementing web-servers) are cross-platform.

Last time I checked, the "the vast majority 3rd party libraries which are not iOS specific" was actually quite tiny.

You'd be surprised. There are a few reasonably well developed server-side frameworks, some of which are already used in production various places.

Also a lot of the libraries which are mostly used in iOS don't have any dependancies on the iOS platform: for instance promise or event emitter implementations etc.

IBM is actually supporting a number of open-source swift projects as well: https://github.com/IBM-Swift.

Between that and painless interop with C/C++, Swift does not feel under-supported on Linux in the least.