As the other commenter said it does seem like Apple might have either straight up removed it or is disabling it in software entirely for now.

Comparison page lists the "Blood Oxygen app" for the S6 through S8, but S9 and S10 both just have "-" for that comparison section like the S5 has.

https://www.apple.com/watch/compare/?modelList=watch-series-...

EDIT: Another difference between them is the Vitals App description removing the blood oxygen reference.

S10: "Vitals app featuring heart rate, respiratory rate, wrist temperature, and sleep duration"

S08: "Vitals app featuring heart rate, respiratory rate, wrist temperature, blood oxygen, and sleep duration"