I often have to go back through my history and look at questions that I’ve commented on to see if the person I replied to has replied to me because they either forgot, or didn’t know how, to use the syntax to target a reply using the @username syntax. Typically this seems to happen with newer users. There have been plenty of times someone has posted a comment directed at me but without using the @username syntax, and therefore I never received any notification. Is it feasible to implement something that would remind users leaving comments (especially new or infrequent users) that when leaving comments directed at others, to use @username, otherwise the person they’re targeting may not see the comment?
This isn't actually necessary in most situations.
If I'm responding to you in the comments on a post you wrote, you'll get notified even if I don't @-mention you.
If I'm responding to you on a post where we're the only two people commenting, you'll get a response even if I don't @-mention you.
The only time the @-mention is needed are those situations where neither one of us owns the post being commented on, or there are more than a few people involved in the discussion.
Code doesn't know if the user who is writing the comment is not using an @-reference because it is not necessary, or because the user forgot to add it.
As reported by the help for comments, the OP who wrote the post is always notified, but also the user who wrote a comment before me is notified, if there are just two users who comments for a post.
As there are cases where a @-reference is not necessary, the code should always remind to the users writing a comment when to use it, which would be a little noisy. There is also the risk that, after seeing the warning all times, the users would simply dismiss the dialog box.