According to this article here: The Outlook AutoComplete list – Outlook | Microsoft Learn

Original KB number:   2199226

 Important

Exchange Online provides some search features through Microsoft Search. Beginning in Outlook for Microsoft 365 Version 2202 (Build 14931.20604), if you’re connected to an Exchange Online mailbox, the suggestions list for the To/Cc/Bcc lines when you compose a message is powered by Microsoft Search. In this specific scenario, only two sections of this article apply: “Enable the AutoComplete feature” and “Remove AutoComplete list entries one at a time.”

This is absolutely asinine if you ask me. There is a second option under this article: Manage suggested recipients in the To, Cc, and Bcc fields in Outlook – Microsoft Support

You have to go to https://myaccount.microsoft.com/settingsandprivacy/

For work or school accounts do the following:

  1. Go to Settings & Privacy for your work or school account.
  2. Select the Privacy tab, and under Data options, expand Manage Contact Search.
  3. You can Reset IndexDownload Contacts, or turn off Capture implicit contacts and contact ranking from my communications.

Then it takes about 48 hours to clear out all your auto-complete contact info that isn’t saved to your contact list.

Note: This settings applies to suggested contact search information for your account and all places where that account information is used in Microsoft (Bing, Calendar, Delve, Excel, LinkedIn, Office Document Sharing, Office.com, OneDrive, OneNote, Outlook Desktop, Outlook Mac, Outlook Mobile, Outlook Web Access (OWA), OWA Mini, People, PowerPoint, Project, SharePoint, Teams for Life, Teams for Work, Windows Search, Windows Share Sheet, Word, Yammer). This doesn’t apply to Xbox.   

Thank you Microsoft for giving us less and less options that we have control over. Fuck the cloud.