Gnome Software "Additional Multimedia Codecs Required" message unclosable

I have the same problem here (on Ubuntu/KDE, if that matters). I can't find out who is trying to get those codecs (ps fx just says it's started by systemd), and I can't find out which package it is trying to install (just a useless sad smiley in a window)

A quick trick to at least stop the notifications: instead of killing gnome-software, STOP it:

pkill -STOP gnome-software

The process will stay alive but won't be able to do anything, so the system won't attempt restarting it, and yet it won't be able to push those notifications.

After you do this, close the remaining notifications and they won't come back (until you reboot, that is). Do pkill -CONT gnome-software to restart it if needed.