Thanks a lot for reporting back!
Then one would only be able to distinguish older versions by the PCB. This is how it still looked on Waveshare’s website on 2021-04-11 (taken from wayback machine):
So there seem to be at least two generations of the PCB of which the older probably only supports brightness adjustment by PWM.
And although the wiki still contains
Question:
How to adjust the backlight?Answer:
You need to remove the resistor and wire the top pad to P1 of Raspberry Pi and control it by PWM.gpio -g pwm 18 0
gpio -g mode 18 pwm
gpio pwmc 1000
gpio -g pwm 18 X（X is in range 0~1024, 0: Darkest, 1024: brightest）
your experience shows that (at least) the current version allows brightness adjustment by software.