Based on following URL all package above 16.x has 21.04 in name. that means updated package only available for 21.04 and above only
URL: http://ubuntu-cloud.archive.canonical.com/ubuntu/dists/focal-updates/wallaby/main/binary-amd64/Packages
Based on following URL all package above 16.x has 21.04 in name. that means updated package only available for 21.04 and above only
URL: http:// ubuntu- cloud.archive. canonical. com/ubuntu/ dists/focal- updates/ wallaby/ main/binary- amd64/Packages