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...