I looked into this topic in some depth recently, as I was suffering from headaches (which turned out to be unrelated to PWM).
I don’t have access to a Librem notebook to test with, but I do know that Intel integrated graphics have baked-in support for screen-dimming using PWM. So, it seems likely that notebooks like the Librems, which use Intel graphics, will use the built-in PWM. Doing it another way would probably involve costly non-standard hardware. I’d wager that most of the PWM-free notebooks available today are using non-Intel graphics. (Though many of those with non-Intel graphics will have PWM.)
Of course, there are good reasons why the Librems use Intel graphics, so it comes down to what’s more important.
A somewhat more realistic prospect might be to use a high PWM frequency. Some people say that a higher frequency stops them having problems with PWM.
Intel graphics do allow for high frequencies to be set, but really the manufacturer should be the one who chooses the frequency, because it needs to be suitable for the actual hardware to ensure stability and avoid damaging it over time.
And then there’s the workaround of just always using it at 100% brightness, which, by rights, should result in no PWM. I tend to do that with my (non-Librem) laptop, since I’m usually in a well-lit location without concerns about battery life.