Using Classic Outlook on Windows for personal email, calendar, and contact management
"View" -> "Current View" > "View Settings" -> "Columns" -> select 'icon' and hit 'remove'
This does NOT fix the problem. It only removes the column -- which also removes the indication that you've replied to or forwarded the email -- indications that are super helpful. If you just remove the column, you don't get any indication re: what you've done with the email. A real fix to the problem would be for Microsoft to remove from their end the mystery yellow envelope that all of the sudden started showing up out of nowhere.