However, I still could not get the compose key working on my freshly installed Ubuntu 20.04.
I use the english (US) keyboard layout, but I need to be able to type Umlauts with the compose key. In Ubuntu 18.04 and 16.04 it worked just fine and I mapped set and used the super key without any problems.
In 20.04 I tried different keys but none of them worked.
The output from
gsettings get org.gnome.desktop.input-sources is:
xkb-options ['compose:rwin', 'eurosign:e', 'esperanto:qwerty'],
sudo vim /etc/dconf/db/local.d/00-input-sources contains