Dear Maintainer,

Adding captcha support would really help deal with the growing problem of bots
spam-subscribing to lists just to get confirmation requests, etc, sent to
mailbomb targets.  This creates a lot of blowback to the list/site admin
emails, wasting time.

Fedora/RH world seem to have done it in the rpm packaging, so the code is there
and doesn't look too hairy.  See (search page for captcha):
https://github.com/pbiering/mailman3-rpm/tree/main

I've tried adding it through local templates and things, but it would seem
impossible without touch files under /usr/lib, which is verboten.  It would
be be done at the packaging level with patches, like Fedora.  Having it
in the anonymous subscription form as well as the user creation form would
be great.

Thank you.