Thanks, Kacee and jsloan. My sleuthing skills have deteriorated, either with age or jet-lag, or both (those 11 hour flights can do that?)

On further investigation, the flight that I was checking, NRT-HNL, while showing an unbookable F cabin on the award redemption page, does not show F as an option for paid flights. For the NRT-HNL trip, this appears to take effect May 1, 2018.