There used to be an "upgrade droplet" action listed in the API docs, but it seems to have been silently removed recently without even a mention in the changelog, and trying to execute an upgrade action now gives a 410 (Gone) error ("The specified action type is no longer available.").

Did I miss a memo somewhere? Is the feature gone for good? What happened to it? And while I'm at it, what exactly did it do to begin with?