Hi MarVdb,
I have got update from related team and they told us it should be an expected behavior that Outlook will leave a label after you deleted the category, If you want to delete the category, you should remove the category from all emails assigned the category.
To remove all, please perform as following:
- Click on the search box and click Categorized, the choose the category you want to delete. Then you will get all emails that assigned the color:


- Then select all (Press Ctrl button and signal click the messages), go to Home > Categorize > signal click the category you assigned. (This will remove the category from all emails assigned the category).
- Then click Categorize > All Categories > Delete the category you want to remove (clearthe checkbox in front of the category) >
OK.

Then the Category should be removed from your category list.
But for the category you have deleted from category list directly, and it shows like "test
(not in Master Category List)" already, to remove it, please try as following:
1. Click on the search box and click Categorized, the choose any category, then you need to modified the name of category in quotes manually to the one you want to delete.

- Then select all messages, go to Home > Categorize > All Categories > clear the checkbox in front of the category > OK.
Finally, please allow me to explain this for you. After you deleted a category from category list, it will leave a default one like "test (not in Master Category List)", it's used to protect users from deleting by accident. Once users deleted the category
accident, user still can recover it via "test (not in Master Category List) label in message". So, deleting category operation become more complex.
Thanks,
Iry