Combat aircraft like the F-16 employs hypobaric pressurisation with supplemental oxygen from a regulator.
However, at what point does the manual call for the pilot to start using the regulator, what altitude?
As a bonus:
What is the maximum altitude one can sustain flight with only hypobaric pressurization under normal operation? Can the pilot fly at say, 20 thousand feet, without ever needing to care about the regulator?
My own reasoning:
It is possible to ascend Mount Everest (approximately 30 thousand feet) without oxygen. Obviously not everyone can do this, but, for a limited time, it seems plausible that the sheer altitude wouldn't be fatal. However, I imagine the manual calls for the regulator a lot earlier than when it becomes dangerous to not have it, for practical and safety reasons. I just have no idea when that point would be, perhaps the manual always states to have the regulator on, that would be the safest option.
Again, I don't know.
The hypobaric pressurization alone would alleviate somewhat the need for supplemental oxygen, so perhaps it is possible to survive, although not comfortably so, beyond 30 thousand feet for multiple hours, thanks to the pressurization.