Side note, I think it was hilarious that Swift was allowed on FOSDEM. Even “free” (as in you probably don’t have to pay for a developer account to use it, *unless you want to ship some binary), Swift remains an Apple product.
You don't need an Apple developer account for Swift on server, Windows and Linux.
You need one only to ship apps on Apple platforms, but that's unrelated to Swift. It applies also to apps written in Objective-C, C/C++, and multi-platform language/frameworks like Dart/Flutter.
The Swift compiler, LLVM, Swift Standard Library, CoreDispatch, the Swift Package Manager and the Swift LLDB debugger are all FOSS and allow you to compile, debug, deploy, sell, buy and ship any binary you want under the terms of the Apache License 2.0.
Deployment of any software (unrelated to Swift) on Apple's platforms is entirely unrelated (and even then, at least on macOS you and any other user can install, sell, buy (...) any binary as desired).
You need one only to ship apps on Apple platforms, but that's unrelated to Swift. It applies also to apps written in Objective-C, C/C++, and multi-platform language/frameworks like Dart/Flutter.