I'm _fairly_ sure that 2021-W07-5T00:00:00Z or 2000-W03-7T01:23:45.678+09:00 would be a valid full ISO date time and offset as well, but I haven't checked the specification.
Luxon will happily parse 2021-W07-5T00:00:00Z and tell me that it represents 2021-02-18T16:00:00.000-08:00, so I think it's valid.
Luxon will happily parse 2021-W07-5T00:00:00Z and tell me that it represents 2021-02-18T16:00:00.000-08:00, so I think it's valid.