Is there any implementation that doesn't require a server-side component? One big appeal of RSS is it's plain old XML that can come from anything over HTTP.
ActivityPub is the part that requires server-side logic. ActivityStreams is a more basic layer that can be entirely static or even serverless, and ActivityPub is built on it.
I don’t enjoy ActivityPub related specs - they give me SOAP vibes - and I’ve struggled with interoperability. (Tried to make my blog act kind of like a Mastodon instance and the learning curve was just way too out there. I’m happy with feeds.)