I often have heard announcements at SFO, indicating that strong headwinds would force them to deny boarding for many pax. Usually, it is on west-bound flights, but not always. (I remember one flight to Japan back in the 90s [old terminal, when the UA F lounge was down in the pit] and they were asking for over 70 pax to volunteer.)

In Jan, I was on LH SFO-MUC, and with headwinds, we were in the air for 11h 37m!