Only checking the Desktop (Win+Mac+Linux) option in supported platforms for packaging results in warnings. Even though this one option should cover all three desktop platforms it still causes a warning to occur when attempting to package for any of them.
Results: A warning message appears stating that you are attempting to package for an unsupported platform.
Expected. To package as normal without any warning messages.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-80686 in the post.