Solution
The problem here is with a stale ppa. There are a few ways to approach this problem, as shown here.
Step(s) Necessary to remediate the original error
Run the following command. Please note that since the ppa in question here is not of the form https://ppa.launchpad.net/x/y/ppa/ubuntu, we cannot straight away use the syntax ppa:x/y as suggested by these solutions: [1], [2], [3].
Important: See this suggestion, which ultimately worked for me.
sudo apt-add-repository --remove https://dl.cloudsmith.io/public/balena/etcher/deb/ubuntu
Once you run this, run the following two commands and you should not find anything as shown in A and B below.
# this should return empty result (as this file must no-longer exist)
ls /etc/apt/sources.list.d/balena-etcher.list
# this should show the default repository for balena (not the ppa)
apt policy | grep "balena"
Once you have verified this, run:
sudo apt update
If you need to remove any GPG security keys alongside, refer to this. This will help in determining the key. And follow this to remove it, if necessary.
# list the trusted keys
sudo apt-key list
# remove the key
sudo apt-key del KEY_ID
And sudo apt update should be working now!
NOTE:
You need to pay attention to two things here to fix this: A and B below.
A: ppa-source-list
When a ppa is added, a ppa-specific file is created under /etc/apt/sources.list.d.
For balen-etcher, there was the following file (/etc/apt/sources.list.d/balena-etcher.list).
# Source: balena
# Site: https://github.com/balena-io/etcher
# Repository: balena / etcher
# Description: Flash OS images to SD cards & USB drives, safely and easily.
deb [signed-by=/usr/share/keyrings/balena-etcher-archive-keyring.gpg] https://dl.cloudsmith.io/public/balena/etcher/deb/ubuntu focal main
deb-src [signed-by=/usr/share/keyrings/balena-etcher-archive-keyring.gpg] https://dl.cloudsmith.io/public/balena/etcher/deb/ubuntu focal main
According to this stack-exchange solution you could also delete this ppa specific file to revert to factory settings (as it was before adding the ppa).
If the ppa is successfully removed the above mentioned ppa-source-list file will cease to exist.
B: apt-policy
Next inspect the apt-policy and search for balena there.
apt policy | grep "balena"
This returned, in my case:
WARNING: apt does not have a stable CLI interface. Use with caution in scripts.
500 https://dl.cloudsmith.io/public/balena/etcher/deb/ubuntu focal/main i386 Packages
release o=cloudsmith/balena/etcher,a=focal,n=focal,c=main,b=i386
500 https://dl.cloudsmith.io/public/balena/etcher/deb/ubuntu focal/main amd64 Packages
release o=cloudsmith/balena/etcher,a=focal,n=focal,c=main,b=amd64
What is a PPA?
Quoting this source.
PPA stands for Personal Package Archive. It provides a way to easily
install application which can not be found in the Ubuntu official
repository on Ubuntu (since the Ubuntu official repo takes a
conservative approach to updates and number of applications it
includes in its official repository with focus on stability over
latest and greatest packages). PPA is one of the coolest things about
Ubuntu. It creates an avenue for developers, packagers and even users
to create their own personal repositories and include their packages
which can easily be added and installed on Ubuntu.
References
