I have created a issue in the pulp_deb tracker to make sure this is not forgotten: Issue #9165: KeyError: 'amd64' when creating publications during 2to3 migration - Debian Support - Pulp
I have also started digging through the code. The error thrown suggests that a ReleaseArchitecture content unit for the amd64 architecture is missing from a repo version being published by the migration. I don’t yet have any ideas why this might be.