A family of Microsoft word processing software products for creating web, email, and print documents.
The Office Insider blog post that Yuhao Li referred to (Easily switch between editing, reviewing, and viewing documents in Word **** ) said that the feature was released in the version 2206 update in late June or early July. To remove it, it's necessary to roll back to version 2205, which I have done here from Microsoft 365 version 2208. After the rollback completes, it may be necessary to restart Word twice to remove the button.
Follow the instructions at https://support.microsoft.com/en-us/topic/how-to-revert-to-an-earlier-version-of-office-2bd5c457-a917-d57e-35a1-f709e3dda841 . The config.xml file described in step 2 paragraph 3 should contain this text:
<Configuration>
<Updates Enabled="TRUE" TargetVersion="16.0.15225.20288" />
</Configuration>
I am submitting this issue to the Microsoft Word development team in hopes that the feature can be fixed to work with document management systems and automated merges.