Same for the Honor Band 5 in the UK until recently, when it has been enabled.

Very cheap device, I see many of them and the Mi Band 4 around here (and Apple watches). Seems to be a saturated market.

Apple has allegedly had spO2 monitoring capability on Apple Watch for years too without making that function available in production. I wonder if there's a similar reason for this. I mean, I can buy a $12 spO2 monitor on Amazon, and most clinics seem to use that exact model... why not let my fitness device do it too?

It is due to the regulatory purview on the difference of calling something a "vitals display device" vs a "vitals monitor"

Source: Spouse, who worked on a field spO2 monitor at her first job and has since spent the next decade in the medical device regulatory industry.

Yeah, they would at least have to do a 510k submittal provided they could show a predicate device existed, and that there were existing standards that they could follow. If they're making an SpO2 monitor that doesn't work like any other SpO2 monitor on the market then the FDA would probably put them in De Novo hell for a very long time.