When I worked at a podcast host I was tasked to implement WebPubSub support to our podcast feeds. This introduced a “push” updates rather than requiring an indexer to constantly recheck the xml file. The idea seeems to not have caught on. At the time I think Google was the only indexer supporting the feature and I’m not sure if that has continued with the switch to YouTube for podcasts
YouTube still supports WebSub (PubSubHubbub)[0] via their own hub[1]. There's very little documentation on how to actually use it though, and it seems like it's been on life support for quite awhile, a lot of small oddities have crept in over time and recently there was extended outages where notifications stopped being pushed. The current documentation isn't even correct, since is specifies an incorrect topic URL (should be /xml/feeds/ rather than just /feeds/, the /feeds/ link is for the base atom feed, and while it actually does "work" if you supply it to the WebSub Hub, it won't send you correct notifications).
[0] https://developers.google.com/youtube/v3/guides/push_notific...
[1] https://pubsubhubbub.appspot.com/
We support WebSub. It helps on the margins, but many feeds do not use it and also it isn't 100% reliable.
Castro recently implemented this I believe.