They still get a request with a Referrer header. But only when the resource isn't cached, so it's not like they're getting notified anytime anyone opens any we

They still get a request with a Referrer header. But only when the resource isn't cached, so it's not like they're getting notified anytime anyone opens any webpage. But they do get a heuristic measure of how much traffic is going to any webpage that loads a font from google.