Managing personal Outlook.com account settings, security, and privacy
I have the same issue, but no one from support was able to give me any guidance or help. Although they assert that the functionality has been removed since May 2021, I was still able to add my Gmail account as of the 26th.
Support claims that the feature is no longer available even though I was still using it up until two days ago. It saddens me to say that a company this big doesn't know what problems they have and do not know how to solve them because I don't believe they have any knowledge of this.