There is a keyboard backlight slider for Lenovo T440s laptop Operating System: KDE neon Developer Edition KDE Plasma Version: 5.13.5 Qt Version: 5.11.1 KDE Frameworks Version: 5.51.0 Kernel Version: 4.15.0-34-generic OS Type: 64-bit Processors: 8 × Intel® Core™ i5-8250 U CPU @ 1.60GHz Memory: 7,7 GiB of RAM
This is going to be a driver bug. Please report it to the kernel developers, if it's still happening for you. I see it with my X1 Yoga gen 7 running the 5.11.4 kernel.