http://releases.ubuntu.com/ubuntu-core/ points to stale images, superseded by http://cdimage.ubuntu.com/ubuntu-core/16/stable/current/

[Steve Langasek]

http://releases.ubuntu.com/ubuntu-core/ contains the original Ubuntu Core 16 images that were released in November 2016. These should at this point be considered obsolete, in favor of the newer images published at <http://cdimage.ubuntu.com/ubuntu-core/16/stable/current/>.

We should remove http://releases.ubuntu.com/ubuntu-core/ so that no one accidentally downloads the older image (which lacks many security fixes applied in later images).

A google search shows no one linking to the old location anymore.