Different planes have different maximum altitudes or ceiling. Less oxygen is not the only limiting factor. Thinner air also allows less lift. If the plane can’t pressurize the cabin, they shouldn’t fly above 10,000 unless they have supplemental oxygen. And other considerations.
The Cessna 172 has a service ceiling of about 14,000 Ft.
I did a test and start climbing to see how high it would go in FS. As expected around 7,000 or 8,000 feet my climb rate started to slow down due to thinner air producing less lift.
The Longitude has a service ceiling of 45,000 feet a several of the larger and new airliners have service ceiling of 41,000 to 43,000 feet.
Notice at the photo below, at 14,000 the 172 only has an IAS of 49 and a maximum climb rate of 105 FPM and at an almost critical AOA.
I continued the climb and it did climb to15,500, but it would not go higher than that. I would imagine a real 172 could also go above 14,000 and maybe make it to 15,500. The engine didn’t stop, but after 15,600 it could not get anymore lift.
I accelerated the sim rate to see if it would climb more or stall, but it didn’t. It just stayed at 15,600 feet. Not sure how a real 172 would do at 15,600, but I expected it to eventually lose lift and stall.
But, close enough for me. As expect, as I climbed higher I lost lift.
Now time to head back down. It’s cold up here and surprised that I haven’t passed out yet from hypoxia.
I did get a stall warning and notice how my airspeed dropped to 38 as soon as I started my turn. I managed to make my 180 degree turn and not fully stull and go into a spiral, but I came real close.