According to the German computer magazine Heise Outlook is now using "Microsoft Synch Technology" without asking the user. There is a Microsoft support article describing the "feature":
Adding Google, iCloud, Yahoo, and IMAP email accounts in the new Outlook for Mac now syncs with Microsoft Cloud to improve sync performance, reliability, and provide additional features available for Microsoft 365 and Outlook.com accounts.
When adding an account there is a checkbox "Sync with Microsoft Cloud":
I can't confirm that an existing account is changed to use Microsoft Cloud:
I'm currently testing direct archiving from Microsoft 365 which automatically uses Microsoft Sync:
I remember when creating the test account a couple of months ago I had to try several times before I saw the "Sync with Microsoft Cloud" and got rid of it.
According to Microsoft it's not possible to change "Sync with Microsoft Cloud" for an existing account. If your account uses the setting then you need to delete the account and add it back!
My emails are private. The emails should only be in the cloud when I allow this. I'm pretty sure that changing this setting for the users and synchronising my private emails to the cloud is against German law. However, Microsoft 365 has never been compliant with German law in the last years.
