You can track the origin of every t address. If it ever went through a z address you decline it. That's why optional privacy doesn't give you fungibility. In Monero every tx is equal. You onlybcan discrimante against the whole.

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Then any user would need to scan that prior to receiving coins because they wouldn't be able to exchange them if the last person or someone many hands ago shielded.
That's exactly what I would do. But I am not in the business of cashing out. I am creating the circular economy for Monero.