Gregory's Server
Replying to something now adds a mention of both the author of the original post and of the reply you're replying to – so both those people get notifications. If you're replying to a reply, that second mention is actually visible.
(side note: there are two kinds of mentions in AP, the one you put into the "tag" field which triggers notifications on Mastodon, and the one you put into the post content as a link that users can click on.)