Missing indicator/tray icons (Steam, Chrome, Dropbox, etc)

The problem was having Unity and its indicators installed. After removing completely Unity and all its indicators the icons are back:

This is the list of the package with "indicator" in the name you should have installed:

$ apt list --installed | grep indicator    

gnome-shell-extension-appindicator/cosmic,cosmic,now 22-1 all [installed]
libappindicator1/cosmic,now 12.10.1+18.04.20180322.1-0ubuntu1 amd64 [installed,automatic]
libappindicator3-1/cosmic,now 12.10.1+18.04.20180322.1-0ubuntu1 amd64 [installed]
libindicator3-7/cosmic,now 16.10.0+18.04.20180321.1-0ubuntu1 amd64 [installed]
libindicator7/cosmic,now 16.10.0+18.04.20180321.1-0ubuntu1 amd64 [installed,automatic]