@trwnh actually what made me think of “extensions of email and newsgroups” was more the object structure, but on second thought that's more an ActivityStream characteristic than an ActivityPub one, although an actual implementation of the C2S part of AP would still fit the bill in some sense.

(Yeah, the lack of usage of DELETE and PATCH surprised me initially, but the fact it would have needed to also define how to propagate them partially explains it.)