I note that this was in a recent commit, so it’s more than likely that this has just not been finished, rather than a bug.
On latest pull from the repo, I added a user field via Customise, a standard text field with “Membership Number” in, and set it to “not editable” after signup.
Going to the signup page shows the usual form (username/password etc), but no box for a field. Trying to register fails, see screenshot below.
The field also doesn’t show on a user’s profile at all, although when set to be editable, is so via preferences.