This feature will partition the server entries in the cache using top frame origin (and also possibly the subframe origin) to prevent sites from using this cache to track users across sites.
Currently, there's a per-profile cache of HTTP auth credentials. Per-origin HTTP auth credentials can be added to the auth cache either by embedding them in a URL, or by obtaining them from a user. Credentials that are successfully used once are added to the cache. Once in the cache, these credentials are automatically sent without user input. This feature will partition the server entries in the cache using top frame origin (and also possibly the subframe origin) to prevent sites from using this cache to track users across sites. Proxy auth credentials will not be affected by this change - proxy credentials entered to load one top level site will automatically be used for others as well.
Docs: N/A
Explainers: No explainer seems necessary, given that this behavior is largely outside the fetch spec.