While you've identified a legitimate bug, the lack of https support, in url_downloadable that function isn't actually used to get the meta-release file. (Wacky I know!) This can be seen by reading UpdateManager/Core/MetaRelease.py's download function.
Please try running do-release-upgrade with the DEBUG_UPDATE_MANAGER environmental variable set e.g. DEBUG_UPDATE_MANAGER=1 do-release-upgrade -c.
While you've identified a legitimate bug, the lack of https support, in url_downloadable that function isn't actually used to get the meta-release file. (Wacky I know!) This can be seen by reading UpdateManager/ Core/MetaReleas e.py's download function.
Please try running do-release-upgrade with the DEBUG_UPDATE_ MANAGER environmental variable set e.g. DEBUG_UPDATE_ MANAGER= 1 do-release-upgrade -c.
Thanks again for your help!