I would think that the original intent of the origin cache sandboxing here, is to disallow different source origins from sharing a target origin, even if that t

I would think that the original intent of the origin cache sandboxing here, is to disallow different source origins from sharing a target origin, even if that target origin wants to be shared. Think "the target origin is an ad-tracking provider domain." I don't see any good way of enabling a target origin to opt into allowing source origins to share caches with it, that wouldn't also reintroduce the privacy leaks. (As, after all, even if the only things malicious-site-X can see in your cache are ad-tech providers' origins that opted into allowing anyone to interface with them, that's likely still enough to fingerprint you.)