So much confusion! I rarely write on this forum, but given the valiant attempts to answer 172510's initial question I think it is merited on this occasion.
JOE-FBS was mighty close;
CS 23.1527 Maximum operating altitude
(a) The maximum altitude up to which operation is allowed, as limited by flight, structural, powerplant, functional, or equipment characteristics, must be established
Effectively the equipment characteristics limit the altitude, more in a bit, firstly let try and close off a few of the lines of thinking.
Handling Issue. Unlikely, if you can stay above stall speed then you may experience some degraded damping characteristics, but these are unlikely to define an altitude limit.
Engine Limitation. These are generally defined by temperature. As temperature varies hugely at altitude defining an engine limitation by altitude seems either overly conservative or inadequate.
High speed buffet. Just no.
Engine restart envelope. This is generally a smaller envelope than the flight envelope in terms of altitude and speed range, so won't be defining your top edge of envelope in any realistic scenario.
Oxygen. Yep. It's the characteristics of this system. In this Canular type system flight is limited to 18 000'. A mask is required
CS 23.1447 Equipment standards for oxygen dispensing units
If certification for operation above 5486m (18 000 ft) (MSL) is requested, each oxygen dispensing unit must cover the nose and mouth of the user.
So without a mask certification limited to 18 000'. Hope this answers your question 172510
