/usr/share/doc/apt/examples/sources.list is in apt 1.4.9.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 | # See sources.list(5) manpage for more information
# Remember that CD-ROMs, DVDs and such are managed through the apt-cdrom tool.
deb http://deb.debian.org/debian stretch main contrib non-free
deb http://security.debian.org stretch/updates main contrib non-free
# Uncomment if you want the apt-get source function to work
#deb-src http://deb.debian.org/debian stretch main contrib non-free
#deb-src http://security.debian.org stretch/updates main contrib non-free
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