For retargeting you would probably have to have the DoubleClick code on your site nonetheless. But at least serving European users you would only be able to act

For retargeting you would probably have to have the DoubleClick code on your site nonetheless. But at least serving European users you would only be able to activate it after a clear opt in via a consent manager. Using marketing parameters (like utm-... in GA or Matomo) or the likes in any other tool still gives you clear marketing performance metrics in your analytics solution of choice, though. And a lot of "pure" tracking tools would - at least currently - not fall under the opt in rules of thew GDPR. But the moment you link them with your advertising profiles (like in GA with the DoubleClick integration) your whole tracking setup also becomes opt in. So you would probably loose at least some share of your traffic in analytics and not be able to clearly do marketing analysis any more. At least not as exact as before. I had clients loosing 80% of their traffic stats in analytics after shifting to opt in.