[karlding]

Rob ter Horst's (The Quantified Scientist) test results compared against a fingertip pulse oximeter—which include at ground level and on flights—seem to indicate that they're okay for detecting whether your SpO2 readings are normal/abnormal. Basically it seems that if you get a one-off abnormal reading then it's possible for it to be a false positive, while you're unlikely to consistently get false positive results.

See the video for the Apple Watch Series 6 [0], and Series 7 [1].

There's also tests for the Series 8 [2], although it doesn't include data collected in a low oxygen environment.

[0] https://youtube.com/watch?v=8HIcwMhEny0

[1] https://youtube.com/watch?v=EI-Bsvo7sHs

[2] https://youtube.com/watch?v=u8XbiWIUW4s