Is a spare mask an absolute requirement when diving solo?
No.
Requirement by who? If you are actually SOLO who would you ask?
It would depend upon the dive, if you are doing solo overhead (which includes deco) then you should probably carry a spare, otherwise not. I do not do solo overhead diving and do not carry a spare mask.
I am a practitioner of Minimalist SCUBA. I do not generally carry multiple items, ponies, spare airs, extra fins, multiple regulators etc. I dive a simple, minimal rig, tank and regulator with a single second stage attached. Above 30 feet, I may not even use a spg, just a J reserve. Below 60 feet I may carry a slung pony. For extreme solo, I have independent doubles set up for solo. My SCUBA rig varies in accordance with the dive, I do not use the same rig or equipment for all dives.
N