There have definitely been a few over the years, especially if you include scenarios beyond just "get notified for replies". Some have been webapps, some have been browser plugins, etc.

Here's some discussion of one recent one:

https://news.ycombinator.com/item?id=21172406

And some more:

https://news.ycombinator.com/item?id=42814917

And HNChat, which went a little further:

https://news.ycombinator.com/item?id=12431724

That said, don't take this as criticism. It's clearly something that some HN users want. If you can make it stick, more power to you!