I've stuck with a specific issue in Mozc package on the latest Ubuntu 21.04, that is of 2.26.4220.100 version. So I figured out I can solve it by downgrading Mozc package to the 2.23.2815.102 version that was working great on 20.04LTS Ubuntu.
Still, when I try to get list of available versions

it shows the only 2.26.4220.100 version available.
But if we look at the complete list, we can see there are lots of older versions available from previous versions of Ubuntu:

So for some reason my apt just doesn't see repositories for other Ubuntu releases. And my question is how to make it do so, so I could just sudo apt install «pkg»=«version»?