Works for Thinkpad X1 Yoga 2G (20JDCTO1WW) (and most probably others) with OLED display
First install inotify-tools. Then create a script around inotify that will be launched upon each boot or through autostart.
sudo apt install inotify-tools
cat <<-EO1 | sudo tee /usr/local/bin/xbacklightmon.sh#!/bin/bash
# https://wiki.archlinux.org/index.php/backlight#sysfs_modified_but_no_brightness_change
# https://wiki.archlinux.org/index.php/Alienware_13#OLED_screen_brightness
# https://gist.github.com/lalten/ce5ac0b1cc4b24a24cc44926255b871d