> I would say that it is a back door, because it would work even if iCloud Photos were E2E encrypted.
Backdoor is defined by the Oxford Dictionary as "a feature or defect of a computer system that allows surreptitious unauthorized access to data."
The system in question requires you to upload the data to iCloud Photos for the tickets to be meaningful and actionable. Both your phone and iCloud services have EULA which call out and allow for such scanning to take place, and Apple has publicly described how the system works as far as its capabilities and limitations. In the sense that people see this as a change in policy (IIRC the actual license agreement language changed over a year ago) , Apple has also described how to no longer use the iCloud Photos service.
One less standard usage is not about unauthorized access but specifically to private surveillance (e.g. "Clipper chip") - but I would argue that the Clipper chip was a case where the surveillance features were specifically not being talked about, hence it still counting as "unauthorized access".
But with a definition that covers broad surveillance instead of unauthorized access, it would still be difficult to classify this as a back door. Such surveillance arguments would only pertain to the person's phone and not to information the user chose to release to external services like iCloud Photos.
To your original point, it would still work with iCloud Photos did not have the key escrow, albeit with less data being capable of being able to be turned over to law enforcement. However iCloud Photos being an external system would still mean this is an intentional and desired feature (presumably) by the actual system owners (Apple).