Suggest that the 18,000ft figure is related to the pressure altitude above which Decompression Sickness becomes an issue. Obviously this is a Cabin Altitude factor, so pressurised aircraft are not normally affected.
Breathing 100% oxygen can provide some protection above 18,000ft Cabin Altitude but only if correctly incorporated with a session of pre-breathing 100% oxygen before ascending. Clearly, this mitigation requires additional equipment to be installed in an unpressurised aircraft which raises issues of weight, cost, maintenance etc.
lm