As I recall, macOS Server used to be available to download for free via a redeem code to Apple Developers enrolled in paid Apple Developer Program. However, after the latest redesign of the Apple Developer portal during mid 2019, I'm not able to find the redeem code anymore.
It used to be possible to get the download code after logging in to the iOS Developer Center and opening the downloads page. There used to be a Get Download Code button which is not there anymore. Here is a picture showing how it looked like.
Now It's only possible to download the macOS server beta, but not the latest released version.
Does anyone know how can I get the redeem code nowadays?