Updated 12 March 2019 with some minor clarifications.. On 21st February 2019, WebKit announced the release of the latest iteration of Safari’s Intelligent Tracking Prevention (ITP), known as ITP 2.1. For a while now, Safari has been targeting cross-site tracking with ITP, first starting with cookies in third-party contexts, then tightening the noose after a number of workarounds emerged, and finally with the latest iteration targeting cookies that were moved from a third-party context to a first-party context.
Updated 7 March 2019 - Added some extra caveats to this solution. Also, be sure to read my article on ITP 2.1, which has far more detail on what Intelligent Tracking Prevention is and how to work with it. Looks like Safari is tightening the noose around browser cookies with the introduction of ITP 2.1 (Intelligent Tracking Prevention). Among other things, ITP 2.1 caps the expiration of client-side cookies to 7 days.