Conversant offers multiple solutions that publishers can utilize to store a unique user id in the first party domain. Similar to IDFA and AAID, the Publisher Common ID (PubCID) is a simple UUID that can be utilized to improve user matching, especially for iOS and MacOS browsers. It is compatible with ITP (Intelligent Tracking Prevention), and is lightweight and self contained. Built with consumer privacy in mind, the PubCID does not sync IDs across domains, so the IDs your domain generates are yours to share with whom you choose. Because the user IDs generated are site specific, there is no degradation of these IDs over time, improving user match rates. Additionally, latency issues generally caused by multiple call outs to various parties are significantly reduced, improving user experience, viewability and earnings. This solution is open source, free to use, and does not require any consortium memberships. 

PubCID for Publishers

PubCID for SSPs