They do mention that subdomains can't be used for tracking in the introduction to Private Click Measurement [1]

But I have to be honest that I can't quite wrap my head around how this is supposed to work.

1. https://webkit.org/blog/11529/introducing-private-click-meas...

Since Apple has access to everyone's emails, they could see that the same email has a unique link in every email and show the user some kind of warning like "This link has a unique tracking ID, do you want to proceed?"

Puts the information and power back in the users hands.

Apples hands*

Stopping tracking by the way of more invasive tracking like extracting content from emails is not a good thing.

Cacheing becomes more challenging, but you could theoretically decode the scheme at the cdn

Stack Overflow has links with user id, I think this will continue to work https://stackoverflow.com/a/70506056/12544391