The subject field was implemented, but AFAIR, it did not propagate to other instances. I don't know whether it was a plugin or a switch, but I do recall turning it off on an earlier #Federati instance because it only displayed on the posting instance.

As for "Private" posts, GS used to have a private post mode (using advisory privacy) ... but it was removed because people thought it was really private, but it was leaky at best. And yes, the private posts were federated across #OStatus.

When I hear about all the "new" halfway-working things in #ActivityPub, almost all of them were tried and discarded in #StatusNet / #GNU_Social before #Mastodon existed.