the use, as far as I know, has always been to disable broken features in older versions of users that do not update. pretty ok and necessary.

but this makes me think they are using it to a/b test or capacity plan for marketing campaigns now :(

I completely disagree this need to be hidden from the end user, it should be FULLY and COMPLETELY transparent what is being disabled, added, or changed. There should be an "about/system-extension" page where a person could go and see everything that is doing or has done, and even optionally disable it if they desire.

There is NEVER a ethical reason for a dark update channel to exist. Altering the functionality of installed software without consent or notification is an act of sabotage, even when done by the vendor. If you want to disable broken features you prompt the user to have them disabled. Worst case (say something that allows malware to propagate or puts them or others at active risk) you disable it and display a message telling exactly what has happened.

I've only seen people that saw the addon being installed unexpectedly.

That doesn't lead to thinking any sites have been hacked.