I have been away for 4 months and when I started Ubuntu again there were 0 updates available. That seems very odd to me. I tried several times and always got that the same result: packages are all up to date. How can that be? Any suggestions?
Ubuntu 20.04.3 LTS - 64-bit - Gnome Version 3.36.8
$ sudo apt update OK:1 http://archive.ubuntu.com/ubuntu focal InRelease OK:2 http://archive.canonical.com/ubuntu focal InRelease OK:3 https://deb.opera.com/opera-stable stable InRelease Paketlisten werden gelesen... Fertig Abhängigkeitsbaum wird aufgebaut. Statusinformationen werden eingelesen.... Fertig Alle Pakete sind aktuell.

sudo apt updatethen upgrade the packages withsudo apt upgrade– moo Jan 23 '22 at 10:52focal-updatesandfocal-securitysources itself. Whatever reason you had for disabling/deleting those sources, it was unwise. Restore them, and don't do that again. – user535733 Jan 23 '22 at 15:38sources.listor did you define a custom path for it? System disables the repositories if they are on custom paths while upgrading certain packages. – Error404 Feb 10 '22 at 10:39