ACME is designed to be extensible by adding new JSON fields, which should be ignored by clients that do not understand them. Unfortunately, some of the earliest ACME clients were intolerant of new fields, which has made it hard to introduce new fields to objects like the one returned from the /directory/ endpoint. Taking inspiration from a Chrome TLS feature we have decided to add a key/keys to directory with randomly generated names in order to dissuade client developers from writing clients in such a way that prevents us from adding new keys in the future.
If you have been directed here because your client broke due to a unexpected key in the directory object with the URL of this thread as its value then you should open an issue on the tracker for the client you use directing the developers to this thread for more information.