I just measured the backlight of my MacBook Air M1 and thought I'd add my results here in case anyone else finds this thread looking for the answer.
I found that it is flicker free at and above 50% brightness. Below there is flicker at a relatively high frequency. 50% and above shouldn't be a problem for anyone as far as the backlight goes, below 50% is high enough that it shouldn't be an issue but I have heard mixed things about ~20 KHz and sensitivity. I hope this helps someone.
Above at 50% brightness. Everything above looks about the same. No flicker detected.
Above at 25% brightness. 19.41 KHz flicker at about 33-50% depending on how you average it.
10% brightness looks similar with similar values.
Above at about 48% brightness, or 7 and 3/4 steps on the brightness scale (or 31 out of 64 quarters if you prefer). 40.8 KHz with about 15-16% flicker.