Oh yeah, I forgot about that. (The one where a staff user can post a highlighted message for all users, right?).
Apparently “whispers” is just what it’s called on other forums too, which I never knew. I kept trying to find a “staff note” or “private note” or “secret note” or “note to other moderators” etc., and never thought to look for a “whisper” (which in non-mod land sounds like a DM). I don’t think I’m the only one confused by this terminology, e.g. Whispers vs Staff Notes, "Staff Notice" ➜ "Notice from Staff"?, Everybody can see "note to staff"!, or the similar personal notes request: Add support for 'personal notes' on posts
Regardless, I’m really glad the functionality is here. I mostly just wanted to add this comment in case anyone else was searching for the same thing, and maybe this will help Google find it.