There used to be a section 23.1447 in 14 CFR Part 23 covering oxygen cannulas and masks. This section does not exist in the current 14 CFR, and I haven't been able to find a new section that covers this same material. Has the FAA done away with the limitation on cannulas being used only below 18,000 feet?
I hope not. A nasal cannula does not increase the inhaled oxygen by much compared to a partial rebreather face mask (the ones with the exhalation valves that close when you inhale). I'd be curious to learn why they feel a cannula is adequate above 18,000 feet or if it was just a clerical oversight.
(Retired physician and former EMT)
Part 23 has been totally reimagined with a completely different certification concept, so many (if not most) specific numeric requirements have been changed to conceptual requirements. But there's a lot of other guidance telling you that cannulas are ineffective above 18,000 MSL.