The log file /var/log/dist-upgrade/main.log would be useful as that contains information about the modification of sources.list e.g.:
2018-04-25 20:01:35,151 DEBUG examining: 'deb-src http://dell.archive.canonical.com/updates/ xenial-dell public' 2018-04-25 20:01:35,152 DEBUG entry '# deb-src http://dell.archive.canonical.com/updates/ xenial-dell public # disabled on upgrade to bionic' was disabled (unknown ** Changed in: debian-installer (Ubuntu) Status: New => Incomplete -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1766787 Title: deb-src entries in sources.list are not updated from xenial to bionic To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/debian-installer/+bug/1766787/+subscriptions -- ubuntu-bugs mailing list [email protected] https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
