/etc/apt/sources.list is currently set up by d-i, but not managed via puppet. This leaves room to all kinds of inconsistencies, e.g. some hosts are using external mirrors (e.g. multatuli is using ftp.nl.debian.org instead of mirrors.wikimedia.org) and crucial apt sources can even be omitted (e.g. planet2001 currently has the security apt sources uncommented (which made me notice this and file this task)).
apt.wikimedia.org and backports are already added via /etc/apt/sources.list.d/wikimedia.list and /etc/apt/sources.list.d/debian-backports.list (and optionally also the experimental component).
I think that at least on Debian systems we should also integrate mirrors.wikimedia.org, security.debian.org and jessie|stretch-updates via sub files in /etc/apt/sources.list.d and simply stub /etc/apt/sources.list with a comment like "# managed via puppetised sub files in /etc/apt/sources/list.d".