Atom/RSS can and did use WebSub (formerly PubSubHubbub) as a Push mechanism:

https://www.w3.org/TR/websub/

And Userlands's RSS specified an <cloud> Element back in RSS 2.0, but the RSS Cloud API only was implemented outside Userland's products first in 2009, in Wordpress, afair, when PSHB already had mindshare.

https://www.rssboard.org/rsscloud-interface