Linux でもデフォルトは 6 https://github.com/apple/swift-corelibs-foundation/blob/e9d8347e64ece3de453d617887af7229f73358d8/Sources/FoundationNetworking/URLSession/URLSessionConfiguration.swift#L73 (edited)

apple/swift-corelibs-foundation

The Foundation Project, providing core utilities, internationalization, and OS independence - apple/swift-corelibs-foundation

1

4:51 PM

どうやらlibcurl(CFURLはlibcurlっぽい)にそのまま渡る模様 https://github.com/apple/swift-corelibs-foundation/blob/2dce19576a6ac1b487a33a0185c8acd3ae911b8f/Sources/FoundationNetworking/URLSession/libcurl/MultiHandle.swift#L69

apple/swift-corelibs-foundation

The Foundation Project, providing core utilities, internationalization, and OS independence - apple/swift-corelibs-foundation

4:53 PM

Apple の Foundation が libcurl ってことはないだろうけれど、API的には同じような振る舞いを期待してるんじゃないかな。