If anyone with a Librem wants to test it for PWM, one simple test is to take a steel rule and wave it back and forth in front of the screen, preferably at a low brightness setting.
If there is no PWM, or high-frequency PWM, you should see a smooth blur of the ruler, but if there is low-frequency PWM, the blur should break up into discrete steps.
(Or you can use your hand instead of a ruler, or measure it with an oscilloscope.)