Hi Stephen,
First of all, just a reminder that this mailing list isn't meant for bug reports. However, this is working as intended so I won't ask you to file a bug instead.
The #enable-new-profile-management flag is still honored but it's an aggregating flag in that it toggles a bunch of things. One of those used to be the #new-avatar-menu flag, which is now removed as of M44, so that's why the button and the menu are still there regardless of #enable-new-profile-management state.
With regards to #enable-fast-user-switching: we agree that going through the User Manager every time you switch profile isn't ideal and that's why we also introduced a faster way: right-click on the button to open a faster user switching view. I suggest you give it a try to see if it solves your use case :)
Finally, I just want to highlight that going back to the old menu was hidden behind a flag for a reason. We needed a way to toggle the features on and off while development was going on but it really is going along Chromium's philosophy to not hang on to multiple ways of doing the same thing. As usual, about://flags carry with them the disclaimer:
WARNING These experimental features may change, break, or disappear at any time.
If you have some other request for a feature or want to report another bug, I suggest you search on
crbug.com for similar things and file a new bug there if you can't find anything that covers what you'd like to report.
Have a great day!