How to downgrade package to the version form previous release of Ubuntu?

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»?

The easiest method would be probably to manually download the .deb file using the "focal" link shown on your screenshot, then install it using sudo dpkg -i ibus-mozc_2.23.2815.102+dfsg-8ubuntu1_amd64.deb (because of dependencies, you will probably also need to downgrade mozc-server and mozc-utils-gui - if installed), and then prevent the packages from further upgrades using either sudo apt-mark hold ibus-mozc (and the same for other downgraded packages), or "Lock version" option in Synaptic Package Manager.