This has to be in cases of rapid decompression. People can live in permanent settlements at 16700 ft (La Rinconada) [0].

[0] - https://en.wikipedia.org/wiki/La_Rinconada,_Peru

Altitude sickness is a thing, so it's not quite that black and white.

People who live at altitude go through physiological changes. My ribcage permanently increased in circumference after living 3000 feet above sea level for over 2.5 years.

See also:

https://en.m.wikipedia.org/wiki/High-altitude_adaptation_in_...