https://www.apple.com/watch/compare/?modelList=watch-series-...
In the past few weeks the oxymeter feature was enabled by a firmware update on series 10. Measurements are done on the watch, results are only reported on a phone.
As of September 9, 2025, hypertension notifications are currently under FDA review and expected to be cleared this month, with availability on Apple Watch Series 9 and later and Apple Watch Ultra 2 and later. The feature is not intended for use by people under 22 years old, those who have been previously diagnosed with hypertension, or pregnant persons.
Sounds a bit ironic but I guess it's for legal reasons.
my guess is this is more like the heart irregularities feature: it’s for the first diagnosis. (a relative of mine actually got diagnosed that way)
The point of the detection features are to notify people who are not diagnosed with the condition to go to the doctor about it possibly get diagnosed with it.
It's not useful for people already diagnosed because they already know they have it, so the notification is just telling them something they already know.