I’m a frontend focused SWE. I suspect the main reason this was abandoned is because it caused technical hell. This has been enabled in incognito mode for a while and has caused a lot of issues with things like fetching profile pictures. For example, if you were on a google.com page that used images hosted on gstatic, they would fail to load because gstatic is “technically” a third party, even though both are Google owned. There are sorta ways around it but the nature of the Internet makes it stupidly annoyingly difficult and causes a lot of stuff to break.
Good idea on paper, but really difficult to implement without breaking the Internet.