They don't explicitly state a warranted capacity for the Model S/X, but I've never heard of an S/X battery with more than ~20% degradation that wasn't considered faulty by Tesla. Whatever Tesla's reason is for not stating a specific capacity, it's not to screw over S/X owners, and I'm sure that 30% capacity loss within 8 years would be considered a fault.

We don't yet know how they stand up over >8 years of service, of course, but there are many examples of very high milage Tesla vehicles (> 200,000 miles) that are still on their original battery and show only modest degradation.