If you've opened the browser console in Google Chrome (since Chrome 76), you might have seen a bunch of warnings in a yellow background related to something called a SameSite cookie attribute that is either missing or incompletely set for cookies set on external domains. If you use Google Tag Manager, especially in Preview mode, you might have seen a warning about the http(s)://www.googletagmanager.com domain. Even though the warning is very prominent, hogging up some prime real estate in the browser console warning, it is, for now, just a warning.
Updated 1 October 2019 With ITP 2.3 it looks like Safari is reducing the usefulness of localStorage as well, so using that as an alternative fix to persistence issues should not be considered future-proof. this solution should not be considered future-proof. 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.