From what I learned there should be an entry in /sys/class/leds/ for kbd_backlight or similar, but there ain't one. The laptop runs Kernel 6.1. I also searched for backlight recursively in /sys, but only found an entry for the display panel.

How can the keyboard backlight be disabled during the startup process?

I found this link that has as possible solution, https://forums.linuxmint.com/viewtopic.php?t=360634

I should have mentioned that I already tried this solution, which is also listed here in the MATE section.
Unfortunately this doesn't work for me on Mint Xfce and even when it works it doesn't suppress the light coming up during fresh boots and while charging according to the Wiki.

I also tried brightnessctl with no results (only the display panel backlight and tiny LEDs are listed).

I assume both brightnessctl and Upower rely on a kernel provided interface below /sys/class/.
Not sure if I need to enable some kind of Thinkpad specific kernel module which might also partially cover my Ideapad Pro?