No, setting does exist. It exists in database, it exists in admin control panel when editing users, it exists in HTML code in both styles and appropriate template variables are set in functions.php. The only place it doesn't exist is UCP because notifications PR removed it. I think it was removed from there by mistake.

Not removing code from overall_header.html is not an option because it will prevent some other code from being removed.

I think best way to go forward is to use this commit and deal with removal (or fixing) of pm popup setting in different ticket.