The intent on the official forums (Discuss) is that extension authors create a dedicated discussion under the Extensions tag, and use that discussion to notify users of updates. The Subscriptions extension is active so users can follow a discussion and get email notifications for new posts.
Unfortunately since the same discussion is also used for support, there can be quite a bit of noise when subscribing to email notifications.
What exactly do you mean by "tag" in your proposal? You mean a new Flarum tag on this forum? How would that work?
Or do you mean an indicator/alert inside of the Flarum admin panel? Mercury should be pretty exhaustive since the 2 primary sources of extensions are Packagist (open-source extensions) and Extiverse (private extensions), both of which Mercury has access to through Extiverse.
If some extensions are missing from Mercury, I suggest you report the issue in the Mercury discussion.
