The Kinetis Design Studio support has been phased out. The latest SDK version 2.4 doesn’t support this toolchain. The SDK builder always builds new SDK packages from the latest SDK version supporting the selected device/board/kit. That is the reason why KDS is not on the list of the available toolchains now.
MCUXpresso web team is working on an improvement that will allow users to generate SDK packages from older SDK releases, if it is needed to get a support for a toolchain or middleware components which are not supported by the latest SDK version. This improvement should be available in early August.
If any user needs any SDK package supporting KDS earlier, then the user can request for the SDK by email sent to email@example.com.