There have been quite a few of them over the years in varying degrees of complexity. I think the most well-known one is probably HN Replies. A simple Github search returns the following results.

HN Notifier

https://github.com/meanands/hn-notifier

Hacker New Telegram Bot

https://github.com/lawxls/HackerNews-Alerts-Bot

HN Reply Notifier

https://github.com/lotsofnoise/hn-reply-notifier

And there have been Show HNs on them as well.

Show HN: I built an open-source notification inbox widget for Hacker News

https://news.ycombinator.com/item?id=36373123

Show HN: HN Reader – Track your conversations across Hacker News

https://news.ycombinator.com/item?id=46240221

I (and probably a lot of other HN users) built my own bespoke HN suite that notifies me on replies, stories that I might be interested in, etc. It just sits quietly on a VPS and sends messages to me over Signal. Never open-sourced it because the code is a dire abomination.

Yours is probably the first Chrome side-panel variation that I've seen though.