How do I mention a user in an announcement reply in Announcements Redesign as an instructor?
You can @mention a user in an announcement reply using Announcements Redesign. Course users mentioned in a reply may receive a notification based on their preferred notification settings.
Reply to Announcement
To reply to the announcement, click the Reply button [1]. To reply to an announcement reply, click the Reply link [2].
Mention User in Reply

Use the Rich Content Editor to enter your reply, links, photos, equations, or media.
To mention a user in your reply type the @ symbol in the Rich Content Editor [1]. A list of course users displays [2].