There is currently no way to distinguish between "customer did not attend", "opportunity attendance updates not supported" and "attendance data was not recorded".

This is resolvable by simply adding https://openactive.io/CustomerAbsent alongside https://openactive.io/CustomerAttended, which allows the booking system to confirm in either case.

And actually renaming these to https://openactive.io/AttendeeAbsent and https://openactive.io/AttendeeAttended to use correct terminology.