On January 22, 2020 we announced in advance that the Microsoft Search in Bing browser extension would be made available through Office 365 ProPlus on Windows devices starting at the end of February. ...
Updated Feb 11, 2020
Version 1.0
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Well, i thought everything is pretty clear in this article. There is no opt-out or forced install planned anymore. There will be an OPT-IN button in admin center. If you don't press it, it won't install on any devices.
If you press it, for now it will only be able to install it on AD-joined devices. That is, if you want this extension to be installed.
Not sure about the scope. I'm guessing before they changed their mind new add-on was included in the main setup and it would disable it from installing via GPO or XML tag, but as it now will be controlled via a toggle in admin center, it probably must have different publishing mechanism. Probably Intune on the backed. And for that to work you have to be managed. So they need to figure out how to deal with unmanaged devices if some customers would WANT this addon installed on unmanaged devices.
The main punchline is that they have BACKED OUT and it is now OPT-IN. Not forced or opt-out.