Ok but who can actually see 240hz?

Like CRTs scan slower than that, and almost no-one could say they could see the pixels being drawn.

Sure you could hear those fuckers, but not see them, unless you had a source of vibration nearby.

Howe much of this is measurable, and how much of it is similar to the Himalayan salt lamp stuff?

The problem isn't the flicker directly (240Hz is above the flicker fusion threshold), it's the phantom array effect when you move your eyes. Static images transform into a series of overlapping images, and this can be annoying and distracting. It's visible well into the kilohertz depending on how strong your saccadic masking is. I can personally see PWM up to about 10kHz (tested using a strobing LED).

AFAIK, all OLED displays have PWM flicker, and for this reason I stick with LCD, where flicker-free displays are available (although PWM flicker is present in some LCD displays too).

Fluorescent tubes used to run at less than 100 Hz before scientists discovered that that was unhealthy for many. Now they run at 30-60 kilohertz.

I can easily see at least some PWM LED headlights flickering in my peripheral vision (which is more sensitive to flicker).

It's not about whether you can "see 240Hz". CRT phosphors have sustain, while OLED does not. The fully-on, fully-off cycle, if infrequent enough, causes some sort of visual stimulation that some brains absolutely do not like. You don't have to see it to feel it.

Sometimes I can tell flickery fixtures because they just "feel more staticky". I don't get visual eyestrain or nausea though.