Pushing instead of polling was done more than fifteen years ago, and various major feed producers and consumers do support it. It was initially known as PubSubHubbub (PuSH), but was renamed to WebSub when adopted by W3C, where it has been a Recommendation for over seven years now <https://www.w3.org/TR/websub/>.

(As for SSE, it’s entirely unsuitable as it would require a persistent connection.)