I personally use only ascii and enforce ascii formatting in CI in every project.
I do this because ascii is easier to type, and newcomers that read the file don’t have to figure out the mental map between unicode and ascii symbols when typing (which is cognitive overhead).
Also, some unicode symbols are problematic to read, for example // with the wrong font almost looks like a /.
I’d personally prefer if the default was ascii, and if that were to happen, as said above, for consistency everything should be ascii then, including Prelude and dhall console output.