Seems to me like a fine topic for discussion at PARAKEET meetings, where I and some other Chrome folks participate regularly.

But for a quick answer to the underlying question: In my view there is still a substantial open privacy question about the technique of proxying some user information to various non-trusted servers. This is the core subject of PARAKEET Issue #11 and came up again in 2021-08-25 PARAKEET meeting. Unfortunately, our (Chrome's) efforts to think about this problem have not landed on any approach that prevents joinability with a publisher's first-party notion of user identity.

Thanks for the quick response, Michael. I will bring it up in the next PARAKEET meeting if you're attending. :)

FYI: WICG/privacy-preserving-ads#42