How can I force `mix format` to enforce the use of parentheses wherever they are possible?

I know that ecto and phoenix export their locals_without_parens and therefore no parens are added to function like from/2 or the ecto schema stuff (take a look at Ecto and Phoenix to see all functions / macros).
Not sure why your partners mix format adds them, if he uses the same config with [:ecto, :phoenix] as imports.

Edit: Which plugins do you use for formatting in VSCode? Just ElixirLS?

That would result in quite weird looking code:

defmodule(Foo) do
  def(foo(arg)) do
    case(arg) do
      :ok -> {:ok, bar()}
      _ -> :error
    end
  end

  def(bar(), do: 10)
end

Hi, its not exactly what you are asking for, but I experienced the same, when formatting with ms code as a shortcut.
At the end we agreed to only use mix format on the terminal.

You can also try to delete the line with the import_deps, which makes it possible to omit brackets in phoenix router and schema files … and more others.