Do you mean ActivityPub (used by the "fediverse" such as Mastodon) or WebSub (which was formerly known as PubSubHubbub).

The former isn't really an equivalent. It requires being always online to receive updates which means that you need to use a service or run your own. It isn't as nice as RSS which can fall back to very occasional polling from my own devices. RSS is also way simpler with a polling rather than subscribing and push.

In my opinion the best option for public publishing is Atom (RSS 2 is also ok) with WebSub support. This provides a really solid set of features.

The only reason I would really recommend ActivityPub is if you need private posts. Otherwise RSS/Atom will be the simpler solution.

> The former isn't really an equivalent. It requires being always online to receive updates which me/ans that you need to use a service or run your own.

ActivityPub is the "push" counterpart to the more foundational ActivityStreams, which works no different than RSS.