There is a list of FreeBSD MLs in the section C.2 of FreeBSD Handbook (https://docs.freebsd.org/en/books/handbook/eresources/#eresources-mail). And freebsd-ports-announce ML is included in it. If someone click the link for the ML, then he is navigated to following URL.
But access to it is redirected to following URL.
As the URL suggests it is page that explain how to (subscribe|unsbscribe) freebsd-ports ML. And as a result, currently user can't (subscribe|unsbscribe|change setting of) freebsd-ports-announce ML
I confirmed clicking link to freebsd-ports-announce ML on FreeBSD Handbook navigates to correct ML page now.