As browsers and platforms deprecate third-party cookies the opportunity for cookies to travel with the user as the user goes from one domain to another is no more, severing the browser infrastructure to measure ad effectiveness.
Why it matters — Because advertising requires cross-context activity, measuring effectiveness requires both contexts to contribute data in order to understand whether an ad was effective. But without third-party cookies traveling with the user from one domain to another, the browser infrastructure to measure ad effectiveness is no longer. The good news, measuring ad effectiveness is still possible without third-party cookies. It just looks a little different.