GDPR isn't about loading external resources, though. It's about tracking. The browser has no way of knowing what kind of tracking is going to happen on the ser

GDPR isn't about loading external resources, though. It's about tracking. The browser has no way of knowing what kind of tracking is going to happen on the server it's connecting to or what jurisdiction that data would fall under. If trackers would provide metadata indicating what kind of tracking each resource is going to do, then browsers could handle it, but trackers would never do that without being forced because they want to make opting out of tracking as painful as possible.