Using New Outlook on Windows for professional communication and productivity
This seems to work in "Microsoft 365 Apps for enterprise" v.2408.
It changes the title in the message, but does not change the title (for me) in the message list, so there is no indication a note has been added unless you're viewing the message directly. If you Forward/Reply then the note _is_ included in the subject and in the included "Subject:..." line; so if the note is not to be shared you'll have to be really careful!