Day 1: A user visits your website at www.your-company.com resulting in a number of cookies being written on .your-company.com, potentially including:
Advertisement click cookies (Google Ads, Facebook, Criteo, etc.)
And many more
The cookies are per default set to expire between 90 and 720 days after, however, ITP 2.1 automatically limits expiry to only 7 days.
Day 2: The same user visits www.your-company.com again. All cookies from day 1 are found and the user is recognized. Any action that the user does is added to the correct user id, click id, etc. Additionally, the 7 days expiry is reset for selected cookies.
Day 10: The same user visits www.your-company.com again. As we are past the 7 days expiry period, the previous cookies no longer exist, resulting in:
A new Google Analytics client ID is generated, treating the user as a new user;
Facebook analytics and click cookie is gone, treating the user as a new user and break attribution for the ad click;
User will need to grant consent to cookies again, as the stored consent is missing;
Any advertisement platform that user click-ids for attribution will not be able to attribute subsequent conversions.