No release file for eoan on old-releases.ubuntu.com [closed]
As of about 7:15 am CST (UTC-6) on 2020-12-14, the Release file exists on http://old-releases.ubuntu.com/ubuntu/dists/eoan/.
I have the same issue, can't upgrade because of this.
I solved my problem by going to the list of Ubuntu mirrors at https://launchpad.net/ubuntu/+archivemirrors
And going through them randomly, until I stumbled upon one that still had eoan: http://de.mirrors.clouvider.net/ubuntu/dists/
I added that mirror instead of old-releases.ubuntu.com, and apt update worked.
Of course, this will probably not last, so I will upgrade to a supported version ASAP.