[Bug 2130273] Re: do-release-upgrade deletes sources.list format entries during update to 25.10

[sem]

I tried to reproduce this.

Step 1: Add the repository via following https://support.mozilla.org/en-
US/kb/install-firefox-linux "For Debian Bookworm or older" instructions.

$ ls /etc/apt/sources.list.d/elastic-8.x.list           extrepo_librewolf.sources.save  tailscale.list  ubuntu.sources.save
extrepo_librewolf.sources  mozilla.list                    ubuntu.sources  xtradeb-ubuntu-apps-plucky.sources

Step 2: Start upgrade process to 25.10 via do-release-upgrade. [NOT THE
GUI]

[... will edit this comment with the results of the upgrade -- since the
browser may need to close during the upgrade process...]

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https://bugs.launchpad.net/bugs/2130273

Title:
  do-release-upgrade deletes sources.list format entries during update
  to 25.10

Status in ubuntu-release-upgrader package in Ubuntu:
  New

Bug description:
  To reproduce:
  1. On Ubuntu 25.04, add a repository in the old sources.list format (https://wiki.debian.org/SourcesList#sources.list_format). For example, I've added the Mozilla Firefox repo via following https://support.mozilla.org/en-US/kb/install-firefox-linux "For Debian Bookworm or older" instructions.
  2. Start upgrade process to 25.10 via do-release-upgrade. In the beginning of the process, the sources.list format repos will be moved into e.g. "mozilla.list.migrate", while the deb822 repos will be copied into e.g. "ubuntu.sources.distUpgrade" 
  3. Finish upgrade. All files with deb822 format will still be there, but mozilla.list will be gone entirely.

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