It is pretty easy if you use the Wim Hof method. Breathe deep and fast for a few minutes, to the point where you get dizzy or feel weird sensations. Then exhale and stop breathing while being fully relaxed. I've done this while hooked to a pulse oximeter and it takes quite a while before O2 actually starts dropping (especially because the effect can be delayed in your limbs), but once it starts you'll pretty quickly run into the regime where a normal oxi will start an alarm because O2 is too low. You can even go below 85% without losing consciousness, because your limbs will desaturate faster than your brain. It's also not uncomfortable, because rising CO2 is what causes breathing reflex, but you dropped its levels far below the threshold by hyperventilating first.

Can’t claim to know for sure, but I’d assume some kind of measurement limitations: the resolution and upper/lower limits of pulse ox are probably calibrated to some medical need, not to detect changes beyond what’s medically necessary

I got a pulse oximeter during COVID and was able to get my O2 levels to spike down to 64% using Wim Hof breathing (for less than a second, ofc).