+1 for discourse
To me mailing list is a bit old. Discourse is more appropriate for a
community like swift has.
Not forget to mention, Rust also use discourse. Rust has two forums, one
for users https://users.rust-lang.org/ and one for discussing about rust
language, compiler, projects https://internals.rust-lang.org/
So swift can have also something like those. With the categories feature of
discourse we can organize our discussions very well