Hoppy
Read TSO-C89 (ETSO-C89 in EASA) and CS-25 for the standards required.
If the aircraft is certified to cruise above a certain altitude it has to be fitted with pressure breathing regulators to meet the required pressure and flow rates.
Yes, and this document says that the max altitude for diluter-demand oxy systems is 40,000 ft. And for simple pressure systems, up to 45,000'. As I said, the normal (N) diluter-demand setting is good for all altitudes, unless you are above 40,000' which this aircraft was not. So your point is?
https://easa.europa.eu/system/files/dfu/CS-ETSO.pdf