I have read stories of WW1 pilots up to 13'000. WW1 pilots didn't have oxygen. I also believe they suffered illness because of it.
WW2 Bomber Crews would often unclip their masks to eat, drink but clip them back again due to hypoxia.
As others have said British needed their masks to talk but iirc both German and American did not as they used throat mikes.
After a battle, when Bader slid his canopy back his squadron would move away from his aircraft a little. He was about to light a cigarette and they didn't want to be engulfed in the ensuing explosion if he had been hit.
Adolf Galland had an ash tray installed in his cockpit.