A new APT sources format "debian.sources" is announced with trixie. The now older "sources.list" format is still supported, but is likely to be deprecated in a future Debian release.
See below:
APT is moving to a different format for configuring where it downloads packages from. The files /etc/apt/sources.list and *.list files in /etc/apt/sources.list.d/ are replaced by files still in that directory but with names ending in .sources, using the new, more readable (deb822 style) format. For details see sources.list(5). Examples of APT configurations in these notes will be given in the new deb822 format.
If your system is using multiple sources files then you will need to ensure they stay consistent.
- https://www.debian.org/releases/trixie/release-notes/upgradi...
"apt modernize-sources" command can be used to simulate and replace ".list" files with the new ".sources" format.
Modernizing will replace .list files with the new .sources format, add Signed-By values where they can be determined automatically, and save the old files into .list.bak files.
This command supports the 'signed-by' and 'trusted' options. If you have specified other options inside [] brackets, please transfer them manually to the output files; see sources.list(5) for a mapping.