I can only speak for tilde.fun, but it's helpful to not have any users currently (^^) and disable outside communication apart from ~/html/ where our users can put static files into.
Most tilde sites don't have public mail servers and only federate mail between them and other tilde servers. See also https://tilde.team/.
Most tilde sites don't have public mail servers and only federate mail between them and other tilde servers. See also https://tilde.team/.