I’ve upgraded to Ubuntu MATE 18.10 and… it was an absolute mess. After having managed to complete the installation, I cleaned a lot of obsolete packages with Synaptic. Among them was gstreamer1.0-fluendo-mp3 that doesn’t seem to be in the 18.10 repos anymore.
The issue is that if I go to the Boutique or Welcome and try to reinstall or uninstall the codecs pack, I get an error message saying that it can’t find this gstreamer package, so it looks like it’s referencing it (at least on my system). Even weirder: I though that this codecs pack was simply matching the package ubuntu-restricted-extras but after removing it and its dependencies, the codecs pack is still shown as installed in the Boutique, so it looks like it references other packages. Anybody knows which ones, so I can remove them manually? (and reinstall them properly to fix this ghost dependency issue.)
EDIT: I reinstalled the bionic version of the gstreamer package and was able to remove the codecs pack, which removed, among others, Rhythmbox, Sound Converter and Subtitle Editor. I was able to reinstall them with no issue but I can’t reinstall this codecs pack because it’s looking for gstreamer1.0-fluendo-mp3. Looks like a bug with the list of packages it installs rather than an issue specific to my system.
EDIT: is it normal that my post has been put in the section for the development version now that Cosmic has been officially released (I made the upgrade after it was)?