Data deprecation will significantly impact the reporting accuracy of any marketing analytics tool that relies on third-party data.  The vast majority of your Google Analytics reports will be fine, because these rely on first-party data from your website. But advertising reports, multi-channel attribution analytics, and customer journey analysis tools will all be affected by a lack of third-party data.  This will make it harder for brands to understand where customers come from, which channels are the most profitable, and how they should spread ad spend.