See my post above #3 again. The update is that the point release for Ubuntu 20.04 LTS will now be, tomorrow (6/8/2020) meaning, we'll all be able to officially upgrade from Ubuntu 18.04 LTS etc to Ubuntu 20.04 LTS if we want to / are ready to, anytime after that date.
However, the way that Plesk Obsidian 18.0.29 announcement is worded, it looks... like, Plesk Obsidian 18.0.29 will only work on fresh, clean Ubuntu 20.04 LTS installs (at present) and that you'll have to migrate from an Ubuntu 18.04 LTS server to a separate Ubuntu 20.04 LTS server, if you wish to run the latest Ubuntu OS & Plesk (which is a lot more work, than simply upgrading the Ubuntu OS 'behind' Plesk, as we have done with all of the previous Ubuntu 18.04 LTS point releases). Fingers crossed that's wrong, but that's how it looks at present