A sibling comment mentioned one service. https://www.hnreplies.com/ is another one that I've used in the past, but it got too noisy (not spam, just frequent emails). I'd prefer an app with push notifications and proper threading.
Like many others, I once tried building an alternative HN frontend with replies caching and push notifications, but the HN API is so barebones/lackluster and poorly documented that I gave up after a few days (basically, for anything that requires a login, you end up having to just scrape the HTML). Every once in a while, there's some talk of improving the API to make it more robust, but I don't think it's a priority for YCombinator.
So old-school scrolling it is :)