It was pointed out to me recently that this doesn't work right in later versions. The problem was I was using NMS to store the DN in the item itself so it could be restored later. That required adding new code every time a new version came out.
I know better methods now; 1.14 introduced the PersistentDataContainer. So I'm using NMS for 1.8 through 1.13, then PDC from 1.14 on. So it should conceivably continue to work as new versions come out unless they make some major changes.
Remove your old config.yml and let it generate a new one.
Changed the way it tracks who has it enabled (not sure why I did it the way I did heh). Also added the ability for you to decide if the functionality will be enabled or disabled by default. You set a boolean value at config path config.default and if you put true, then the no name display functionality will be automatically enabled for anyone who didn't explicitly disable it. If you set it false, then it will default to showing names.
If the person uses /nodn to toggle, that takes precedent over the config default.