Summary: Apple introduced PCM [1], and to keep people from using it for cross-site tracking it limits the bits available to a single site (as defined by the PSL). If shop-a.retail.example and shop-b.retail.example are completely separate, and don't want to compete for bits, Apple will still treat them as a single site unless retail.example is on the PSL. Being on the PSL is a big change (partitioned cookies, etc) but could be appropriate for different shops.
FB issued guidance suggesting domains like retail.example consider getting themselves added to the PSL, and now the PSL (a volunteer project) is getting a lot of requests. The PSL project has put these requests on hold, and asked FB and Apple to work this out. FB is talking to Apple in https://github.com/privacycg/private-click-measurement/issue...
[1] https://webkit.org/blog/11529/introducing-private-click-meas...
1. Multi-tenant domains that probably should've always been in the PSL (ex. to provide cookie silos) but are only realizing now that they should be in it due to the arrival of PCM.
2. Sites that want to abuse an eTLD to do something like give all users on their social network a custom subdomain so that they're not polluting the same pool.
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I think it was actually reasonable for Apple to consider the PSL as it's basically the most comprehensive eTLD list that we have and would allow them to match browser behavior.
The problem now is that case (1) is sending a bunch of requests at once as something will now actually break for these sites. Before now it was really just them being lax with security and not considering that cookies should be siloed. This isn't a unique situation btw, PSL also saw a large increase in inclusion requests when LetsEncrypt added rate limits based on eTLDs.
(2) is obviously bad and there's really no other justification for these sites being in the PSL.
Therefore I think it's reasonable for PSL to deny inclusion requests that are solely for PCM reasons.
This all being said, the PSL is a massive hack [1] and really needs to be replaced by something else. It probably is about time for these companies to invest in a replacement.