You can continue using Plug.Static by doing something similar as in this article.
We have recently done this in an app, create the static pipeline, then combine it with a plug that does the authentication.

  pipeline :static do
    plug :accepts, ["html"]
    plug MyAppWeb.Plugs.Authenticate

    plug Plug.Static,
      at: "/help",
      from: {__MODULE__, :pages_path, []}

    plug :needs_index
  end

  scope "/help", MyAppHelpWeb do
    pipe_through :static
    get "/*path", HelpController, :index
  end

the :needs_index plug just checks to see if we are asking for a path instead of a file, and tags on index.html.
The controller renders a 404, because we only get there if the file doesn’t exist.