All errors found in 20.12 have been fixed and the only newness is a new background song.
But the great novelty is that a snap version is available for all GNU/Linux that supports it (only for amd64, armhf and arm64 architectures). You can install it from your package installer or following the instructions of https://snapcraft.io/ultrastar-worldparty. You can start it from the desktop shortcut or from console using snap run ultrastar-worldparty (or snap run ultrastar-worldparty-debug for the debug version). The user installation directory is ~/snap/ultrastar-worldparty/current/.ultrastar-worldparty, and to add songs must create a songs directory or edit the config.ini and add all directories as you want using options: SongDir1, SongDir2, etc.
If you won't use the snap version and use a Debian/Ubuntu, you can download the deb package for amd64. And of course the usual Windows version is available too.