Using New Outlook on Windows for personal email, calendar, and contact management
I am using Classic Outlook and I don't have the functionality anymore either. I worked with a Microsoft tech over the phone months ago and when he made some changes to my Outlook (reinstalling it), I didn't have it. I don't have the bandwidth to figure this out because I tried all the suggestions.
Now I just set up separate files with emails I need to follow up and then enter a task. Yes, it's double work but it's my workaround.
Most likely, within Classic Outlook, your account was originally configured as a POP3 account, which has full support for Flagged Items and Reminders. When your account was reconfigured by the Microsoft tech, it must have been configured as an IMAP account instead which only has limited flagging support.
See: Follow Up Flags and Reminders for IMAP accounts - MSOutlook.info