Using Classic Outlook on Windows for personal email, calendar, and contact management
Yes it sort of works - it worked for some folders for me (when I removed the letter, it stayed in place) - but for other folders it moved them back in alphabetical order whenever I rename them...
If you want to try:
Arrange all folders as you'd like to see them appear by renaming them, adding 1 letter in front of each: (a., b., c.,) - Outlook will then automatically rearrange the folders in your chosen order. Then, click on "rename" and remove the letter at the front. It might stay where you placed the folder!