The quiestion arose elsewehere, but I can't find the thread now.
It was discussed here:
Thread 195795
You'll find that Russ said it was on his todo
This has now been implemented - but not in production. (Coming in the new upcoming update, as you have probably heard many times by now!).
If you reply or quote someone in a thread, they will receive a notification, and the notification will take them right to your post in the forums.
( Ref : Yesterday's post on X/Twitter Twitter thread of Work In Progress screengrabs )
Note : When beta testers get going on the new build, they will generate these notifications when replying to forum posts, but the users of the current production site will not - so a possible source of confusion until the beta is promoted to production.
As an aside, the notification system is much improved too, you can view notification and delete notifications in a pop out side panel without losing your current page.