I just had to learn that /my/group/messages/My-Group does not work as expected.
Naively I was sure, that it would redirect to /u/thoka/group/messages/My-Group, which it does not.
Couldn’t the redirection /my to /u/{current_username} just work for any URL inside the user namespace?
Move this feature request to Contribute > Bug, if my assumption is the desired behavior.