OK I consulted my LLM on that definition. It says to me if its as "public upfront design decision" in a software it is not a backdoor by definition. But I can see your case here. If you come from classic ecash it might feel like a backdoor because its privacy is weakend on one side by design. If you come from PayPal, Visa or SEPA system it still feels like a big privacy improvement .. and would be so much better what mainstream uses right now.