I didn’t claim that it’s impossible to imagine ways that this could be done. I was pointing out that no such methods are actually in common use, which means that when people think about validating their age online, they are concerned about their privacy.

There is a technical possibility to do that. There is also a technical possibility to leak identities.

Sure, there are ways. There are ways to watch every single American all the time; China is trying that with their citizens already. Doesn't mean we should, or will, or that it will ever make sense.

PH is a business, and the overhead of adding this system, on top of the PII, compliance, and access risks that come with it, doesn't make business-sense.

What costs more? implementing a sweeping system with tons of risk, or blacklisting one state?

The overhead of adding a decently designed and broadly used age verification mechanism is almost nil. It won't show up even as an operating expense.