I mean personally I think they should do client-side feature detection and be back to being standards compliant and not creepy. The only reason why I'd consider such a flag is because they optimize the payload server-side to return a certain a/b test, but even with that they could do the default version first, do feature detection, and then set a session cookie for that domain only that loads the a/b test.

My other Thought was that they test a feature that is implemented across Google's properties, e.g. something having to do with their account management.