Love the idea of reprimanding uncool behavior, but I don’t think this can truly be anonymous. The recruiter will surely have some idea of who sent the message.
Nor does it need to be anonymous. If someone ghosts you, there's no harm and nothing to be lost in politely letting them know it's not cool, and you'd be reluctant to work with them on any future positions.
I have done this before, and received apologies. In general I don't have a lot of time for people wanting to do things anonymously.
I'm glad you've gotten a good response. I called out a recruiter on the phone once for putting me through a bait and switch interview process. I got nothing but incredulity for my troubles. Still felt good though, and I don't care if I ever see or hear from that company again.
I have done this before, and received apologies. In general I don't have a lot of time for people wanting to do things anonymously.