Default Ubuntu /etc/apt/sources.list

That depends on which version of Ubuntu you have installed. One way to get an accurate copy would be to deploy a new Linode with the same version of ubuntu and copy over the /etc/apt/sources.list file.

I've included an example sources.list from my own Ubuntu 19.10 Linode. You could try replacing "eoan" with the name of the version of Ubuntu on your Linode, but depending on the version it may not work.

# 

# deb cdrom:[Ubuntu-Server 19.10 _Eoan Ermine_ - Release amd64 (20191017)]/ eoan main restricted

#deb cdrom:[Ubuntu-Server 19.10 _Eoan Ermine_ - Release amd64 (20191017)]/ eoan main restricted

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://mirrors.linode.com/ubuntu/ eoan main restricted
# deb-src http://mirrors.linode.com/ubuntu/ eoan main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://mirrors.linode.com/ubuntu/ eoan-updates main restricted
# deb-src http://mirrors.linode.com/ubuntu/ eoan-updates main restricted

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://mirrors.linode.com/ubuntu/ eoan universe
# deb-src http://mirrors.linode.com/ubuntu/ eoan universe
deb http://mirrors.linode.com/ubuntu/ eoan-updates universe
# deb-src http://mirrors.linode.com/ubuntu/ eoan-updates universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu 
## team, and may not be under a free licence. Please satisfy yourself as to 
## your rights to use the software. Also, please note that software in 
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://mirrors.linode.com/ubuntu/ eoan multiverse
# deb-src http://mirrors.linode.com/ubuntu/ eoan multiverse
deb http://mirrors.linode.com/ubuntu/ eoan-updates multiverse
# deb-src http://mirrors.linode.com/ubuntu/ eoan-updates multiverse

## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://mirrors.linode.com/ubuntu/ eoan-backports main restricted universe multiverse
# deb-src http://mirrors.linode.com/ubuntu/ eoan-backports main restricted universe multiverse

## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu eoan partner
# deb-src http://archive.canonical.com/ubuntu eoan partner

deb http://security.ubuntu.com/ubuntu eoan-security main restricted
# deb-src http://security.ubuntu.com/ubuntu eoan-security main restricted
deb http://security.ubuntu.com/ubuntu eoan-security universe
# deb-src http://security.ubuntu.com/ubuntu eoan-security universe
deb http://security.ubuntu.com/ubuntu eoan-security multiverse
# deb-src http://security.ubuntu.com/ubuntu eoan-security multiverse

# This system was installed using small removable media
# (e.g. netinst, live or single CD). The matching "deb cdrom"
# entries were disabled at the end of the installation process.
# For information about how to configure apt package sources,
# see the sources.list(5) manual.

Thank you, that's just what I need.

Out of curiosity, how do the Linode mirrors differ from the regular Ubuntu mirrors?

The main difference between using Linode's package mirrors and regular Ubuntu mirrors is speed. This is because we have mirrors set up in each data center. Otherwise, our mirrors are the same as the Ubuntu mirrors.