Scoop places a [new] tag next to comments that have been posted since the last time you refreshed the page, so that you can just Ctrl-F. So there have been popular sites doing this for 20+ years, e.g. Kuro5hin and DailyKos. HN just hasn't had features like this due to being limited by single threaded compute for various reasons. Although if the comments were served via an API, this could now be down in the front end with just the timestamp of when each user last refreshed each page.