[keyringlight]

IIRC there are systems to pre-generate tracking numbers that get allocated to and used by larger customers of couriers, so they can do more of the dispatch setup internally without thousands of "book this shipment and give me a tracking number" calls to the courier and they reconcile it later. That could at least get you to "this is a valid tracking number" stage, but before it's within the courier's logistics they've no way of knowing if it's active or what package it belongs to, which doesn't seem like any better of a customer experience.

If you wanted to make it universal it'd be a higher hurdle for small volume shippers, and it wouldn't surprise me if there's challenges to be overcome with a large number of outstanding 'valid but not used' tracking ids.