From what I understand, this is more because having that low of oxygen at rest is highly likely to be a symptom of a deeper problem, not because the 90% oxygen by itself is a huge imminent problem. Afaik you can go days/weeks at that level and be mostly fine if you are otherwise healthy. For example, this paper suggests that some people who live at high altitudes have perpetual SpO2 of <90%: https://www.thelancet.com/journals/langlo/article/PIIS2214-1....

For those of us living near sea level, temporary drops below 90% should be ok if you are otherwise healthy (up to a certain point of course), because as someone else in the thread suggested, they will rebound to >=95% as soon as you return to rest/normal breathing.

(Disclaimer: I'm not a medical expert, but have had to study breathing a fair amount. Happy to have corrections from anyone with more knowledge on this!)