OBOGS is standard on the Hawk T Mk 2, but sensibly there is a back up compressed O2 supply that would get the pilot/s down to breathable altitudes.
Interesting how often 'dramatic' statements like 'so-and-so fleet grounded' appear here though.
Any manufacturer/operator is being prudent by ensuring the safety of both the company (vicarious liability, duty of care etc etc) and operators (crews and pax when carried).
It's not a particularly dramatic event, it's actually putting safety first - for whatever the reason (I'm a cynic).
I suppose the worst thing about hypoxia is its insidious nature, so the (cabin) or altitude limit is eminently sensible.