This problem was reported three times. 2014, 2016, and 2020. It is listed in a few postings. It was fixed by Cristobal Palmer by modifying listinfo.html template to add the CAPTCHA. Why is this periodically breaking? It needs another fix now.
@Sherwin_Dubren Thank you for bringing up this issue. We are currently piloting a replacement of Mailman that aims to solve this issue. I am opening a ticket on your behalf; please check your email for details.
It is now over two weeks since this ticket was opened and I still have not seen any fix or notice about it.