downgrade_firmware.sh

#!/bin/bash

list_packages()
{
    echo "libraspberrypi-bin libraspberrypi-dev libraspberrypi-doc libraspberrypi0 raspberrypi-bootloader raspberrypi-kernel raspberrypi-kernel-headers"
}

target_version()
{
    echo "1.20190401-1_armhf.deb"
}

fetch_archives()
{
    local base_url="https://archive.raspberrypi.org/debian/pool/main/r/raspberrypi-firmware"

    mkdir -p /tmp/downgrade_packages
    for package in $(list_packages); do
        wget -P /tmp/downgrade_packages "$base_url"/"$package"_"$(target_version)"
    done
}

install_archives()
{
    dpkg -i /tmp/downgrade_packages/*.deb
}

set_selections()
{
    local op=$1
    local package

    for package in $(list_packages); do
        echo "$package $op" | dpkg --set-selections
    done
}

argument=$1

case $argument in
downgrade)
    echo "downgrading kernel..."
    fetch_archives
    install_archives
    set_selections hold
    echo "downgrade complete! reboot for changes to take effect."
    ;;
block)
    echo "adding package holds..."
    set_selections hold
    echo "block complete! firmware packages will be held at current version."
    ;;
unblock)
    echo "removing package holds..."
    set_selections install
    echo "unblock complete! new firmware packages are now installable via apt-get."
    ;;
*)
    echo -e "supported arguments:\n* downgrade: downgrade and hold firmware to version $(target_version)\n* block: hold firmware package updates\n* unblock: allow upgrade of firmware packages"
    ;;
esac

Thank you for this.