It is an abuse of Chrome's position in the marketplace. Google is using their powerful position to give themselves tracking capabilities that other online players can't access. It is a major competitive advantage for Google.

can't alternate browser makers who base on chromium simply disable that portion? like, I expect identifying users was a key business concern in moving Edge to Chromium. Is there something (other than work) preventing them from making it so it'll report back to microsoft-owned domains instead?

I'm using Vivaldi on MacOS and it doesn't send this header. I'm sure others like Brave don't send it either.

Is it because Google's webapps will have their own a/b tests which use experimental features only available in Chrome perhaps?

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.

Isn't this what cookies are for?

Cross-site cookies are soon getting blocked by Chrome starting Chrome 80 if I'm right (whereas this header isn't)

So they build a personal back door to a feature that they've chosen to remove for everyone else? Because of it's potential for abuse, yet the very same company is somehow abusing it in a way more sinister way. Antitrust can't come soon enough.

Chrome will only block cross-site cookies that don't use HTTPS and the SameSite=Lax flag. It's easy for trackers to user HTTPS and SameSite=Lax. This Chrome change is mostly intended to protect against Cross Site Request Forgery (CSRF) attacks, not to block trackers.

I can think of a hundreds reasons why they do this. It doesn't make it right in any of those.

Err yeah, because it adds loads of data that can be used to track you.