mailman3 supports using a whoosh (default, pure Python) backend for full text search or a supposedly faster/better xapian backend
See https://django-haystack.readthedocs.io/en/v2.4.1/installing_search_engines.html#xapian and https://docs.mailman3.org/en/latest/config-web.html#enable-full-text-search-required
The xapian-haystack package in Buster is broken, see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=956653