System update corrupted system

[Ralf Mardorf]

https://packages.ubuntu.com/bionic/mate-panel
https://packages.ubuntu.com/focal/mate-panel
https://packages.ubuntu.com/groovy/mate-panel
https://packages.ubuntu.com/hirsute/mate-panel
https://packages.ubuntu.com/impish/mate-panel

mate-panel is in universe, too.

Btw. not only panels from universe get a little bit broken, way more
relevant software, such as browser, e.g. falkon tend to break
completely within a LTS release cycle.

When using all official repos, then even a LTS is absolutely unstable.

An official flavour is just a flavour, it's not Ubuntu.