I was going on a Wired article that said there were. However, you are correct, I couldn’t find any.
From my perspective, GPC is mostly smoke and mirrors, along with SPF, DKIM and DMARC for email, each of which, when implemented, was the Next Great Thing to block spam. Have you noticed how effective they are? Google has just announced that starting next month gmail will block all incoming email that does not implement all 3-SPF, DKIM and DMARC. We’ll see if it makes a difference.
GPC says to websites “please don’t track me”. Sites do not have to honor it, and the ones most likely to honor it already have similar policies. The ones that don’t currently aren’t going to change. So the choice of Apple’s iCloud Private Relay, which hides who you are, or GPC, which reveals who you are, but says “don’t use the information I have just given you” leans towards Private Relay.
I agree it would be the best of both worlds if Apple implemented GPC, but GPC is much less than meets the eye. It does NOT require sites to honor it, unless the sites are hosted in California (but not if they are hosted in the Maldives). And I suspect the reason Apple has not implemented it is they feel it would provide a false sense of security.