A script that helps downgrading Debian-based systems

aptitude-downgrader.sh

#!/bin/bash
deps() {
	echo "Missing dependencies, run:\n  sudo apt-get install aptitude grep"
	exit 1
}

if ! command -v aptitude &>/dev/null; then
	deps
fi
if ! command -v grep &>/dev/null; then
	deps
fi
if [ "$EUID" -ne 0 ]; then
	echo "Please run as root"
	exit 1
fi

# Update package information and downgrade dash first
sudo apt-get update
apt-get download dash
sudo dpkg -i /tmp/dash_*.deb
rm dash_*.deb

aptitude keep-all # delete all previously scheduled actions

manual_packages="$(apt-mark showmanual)"
get_package_state() {
	# if the package is manually installed, return "&m", which is the aptitude syntax for manual
	# else return "&M", which is the aptitude syntax for automatic
	match=$(grep -q "^$1$" <<<"$manual_packages")
	if [[ $? -eq 0 ]]; then
		echo ""
	else
		echo "+M"
	fi
}

get_package_policy() {
	local package=$1
	# Get the version table for the package
	version_table=$(apt-cache policy $package)
	# Get the candidate version for the package
	candidate_version=$(echo "$version_table" | grep 'Candidate:' | cut -d ' ' -f 4)
	if ! apt-cache madison "$package" | grep -q "$candidate_version"; then
		candidate_version="(none)"
	fi
	# Extract the priority of the candidate version
	candidate_priority=$(echo "$version_table" | grep "$candidate_version" | tr ' ' '\n' | tail -n 1)
	# Return the array
	echo $candidate_priority $candidate_version
}

# Get the list of installed packages using the current status
installed_packages=$(dpkg -l | grep '^.i' | cut -d ' ' -f 3)

scheduled_actions=""
# Loop through each installed package
for package in $installed_packages; do

	# Get the version of the package provided by the release with the highest priority
	policy=($(get_package_policy $package))
	package_priority=${policy[0]}
	package_version=${policy[1]}

	# Check if the package is not provided by any release
	if [[ "$package_version" == "(none)" ]]; then
		# If not, set it as deinstall
		echo " - Keeping package $package as it is not provided by any release"
		continue
	fi
	# Check if the package replaces other packages
	# -f 2- means that we are cutting the first field and keeping the rest
	# -f 2 is used because after a ',' there is a space, which will be the first field
	replaced_packages=$(apt-cache show $package | grep "Replaces:" | cut -d ':' -f 2- | tr ',' '\n' | cut -d ' ' -f 2)

	# take the package with the highest priority
	largest_priority="$package_priority"
	version_to_install="$package_version"
	package_to_install=$package
	for replaced_package in $replaced_packages; do
		replaced_policy=($(get_package_policy $replaced_package))
		replaced_priority=${replaced_policy[0]}
		replaced_version=${replaced_policy[1]}
		# If yes, compare the priorities of the replacing package and the replaced package
		if [[ "$largest_priority" -lt "$replaced_priority" ]]; then
			# If the replacing package has a larger priority than the current largest
			# priority
			largest_priority=$replaced_priority
			version_to_install=$replaced_version
			package_to_install=$replaced_package
		fi
	done
	if [[ $package_to_install != $package ]]; then
		echo " - >> Found name mismatching: $package <-> $package_to_install! deinstalling $package (priority $package_priority) and installing $package_to_install (priority $largest_priority)"
		scheduled_actions="$scheduled_actions $package-"
		echo " - Package $package is replaced by $package_to_install"
		package=$package_to_install
		package_version=$version_to_install
	fi
	package_state=$(get_package_state $package)
	if [[ $package_state == "+M" ]]; then
		echo " - AUTOMATICALLY installing $package=$package_version"
	else
		echo " - MANUALLY installing $package=$package_version"
	fi
	scheduled_actions="$scheduled_actions $package=$package_version$package_state"
done

echo "$scheduled_actions" >aptitude_command.sh # debugging

aptitude install $scheduled_actions

# Apply the selections
echo "Applied package selections"

if [[ $? -ne 0 ]]; then
	echo "An error occurred while resolving dependencies"
	exit 1
else
	echo "Resolved dependencies"
fi

Use it at your own risk.
It leverages aptitude to downgrade your apt-based distribution.

Usually, downgrading is made difficult by several factors:

1. package name mismatching
2. new packages not available in the previous version
3. installation order
4. configuration files changed

This tool solves only the point 1, which in turns make easier the next points.

For point 2., it starts aptitude and leaves the user the ability to choose the best solution to fix the incompatibilities. I reccommend to avoid solutions that don't satisfy some dependency and to prefer solutions where only packages of the downgraded version is available. Aptitude is usually able to find a solution, but you may need to start it interactively by pressing e and fix the dependencies manually.

For point 3., there is no easy way to fix. The order of installation is decided based on the dependencies of the packages, disregarding configuration and pre-installation/post-removal scripts. This script allows to avoid many issues by first downgrading dash. However, I'm still experiencing issues due to libssl3.

Point 4 can only be left to the luck of the package developer. This is usually not a problem