At first I want to say here a big THANKS for all this super useful information. This really helped me to get rid of an old and outdated oibaf PPA installation in conjunction with Kubuntu 20.04 LTS.
The above given information solved the problem for me but I should add here most likely some additional important stuff.
Keep a BIG attention when removing the oibaf files through synaptic. As far I remember, I correctly followed the instructions but when I clicked on the Apply button I got an error message. Then I simply tried it again and at the second time it started to remove the files. Shortly after that I realized that for whatever reason the whole process took much longer then expected. I also saw that there were much more files removed then just the oibaf ones.
In the end it turned out that the whole KDE plasma desktop was also uninstalled. As a result, I had after a reboot no longer any GUI available, it always booted to the console. This was not so grave, because it was still possible to login with username and password. And I had also still internet access via network cable (aka LAN). But the solution was actually quite simple, - the desktop environment had to be reinstalled.
So, when you are on Kubuntu then just perform:
sudo apt-get install --reinstall kubuntu-desktop
Or alternatively, when you are on Ubuntu:
sudo apt-get install --reinstall ubuntu-desktop
This should reinstall all default packages. To some degree this does act like a “repair installation” on Windows. I didn’t know that those arguments exists. This was really helpful in that situation.
Unfortunately I tried first an other approach. I had the glorious idea to make a distro upgrade from CLI. So I purged all remaining third-party PPAs with
ppa-purge and started the upgrade process to Kubuntu 22.04 LTS.
The upgrade process worked but a release upgrade is not a reinstall. So I ended up again at the console. And this time I had for whatever reason no Internet access. So the whole situation was even worse. After a more closely look I saw that the Ubuntu “Network Manager” was not installed. I download the needed files at an other computer and then I installed them manually:
sudo dpkg -i network-manager_1.36.4-2ubuntu1_amd64.deb libbluetooth3_5.64-0ubuntu1_amd64.deb glib0_1.18.6-1_amd64.deb libndp0_1.8-0ubuntu3_amd64.deb libnm0_1.36.4-2ubuntu1_amd64.deb libteamdctl0_1.31-1build2_amd64.deb
Additionally I installed also
sudo dpkg -i net-tools_1.60+git20181103.0eebece-1ubuntu5_amd64.deb
To my great relief this was enough to get it again working. After a reboot to the console I had again Internet access.
Finally, I applied then the above mentioned reinstall command of the kubuntu-desktop. This reinstalled the whole GUI while preserving all settings and configs. So after a new reboot everything was again exactly on its place, nothing got lost.
And by the way, the oibaf PPA can be absolutely recommended. It is a GREAT possibility to get the newest Mesa builds. It provides you with the latest functionality and compatibility regarding any graphics card. But yeah, the end-user must give attention to the fact when it becomes EOL. It is the sole responsibility of the end-user to look about that. That should be clear from the beginning.
I wasn’t aware of the fact that it is quite complicated to uninstall that PPA when the Ubuntu release is no longer supported and all corresponding files are deleted from the PPA.
Whatever, I have learned again A LOT! And that’s the nice part of that little adventure.