Which many may know as Pubsubhubbub, with only minor changes and a new name. (Which is a good thing: no need to make up a new protocol if there is already one in the wild that works)

WebSub literally does the same thing, except ActivityPub cuts away the middle layers, removing some redundancy on the way. It makes ActivityPub less of a protocol hodge-podge than OStatus, which has the same drawbacks and benefits as any introduction of a dependency into a protocol or software.