New development: https://webkit.org/blog/8613/intelligent-tracking-prevention-2-1/
In particular:
--------- WebKit implemented partitioned caches more than five years ago. A partitioned cache means cache entries for third-party resources are double-keyed to their origin and the first-party eTLD+1. This prohibits cross-site trackers from using the cache to track users. Even so, our research has shown that trackers, in order to keep their practices alive under ITP, have resorted to partitioned cache abuse. Therefore, we have developed the verified partitioned cache.
When a partitioned cache entry is created for a domain that’s classified by ITP as having cross-site tracking capabilities, the entry gets flagged for verification. After seven days, if there’s a cache hit for such a flagged entry, WebKit will act as if it has never seen this resource and load it again. The new response is then compared to the cached response and if they match in the ways we care about for privacy reasons, the verification flag is cleared and the cache entry is from that point considered legitimate. However, if the new response does not match the cache entry, the old entry is discarded, and a new one is created with the verification flag set, and the verification process starts over.
ITP currently does this verification for permanent redirects since that’s where we see abuse today. ----------
It's not clear to me if the permanent redirects are in a partitioned cache though. Either way, this doesn't affect Tor too much given that we don't save history.
Although it does bring up a simple case that e could implement with no problem: never remember a permanent redirect.
-tom