I would simply define a route for it and dynamically render it by environment.

Reviving this thread with a solution on how to achieve this behavior with a Plug, since I haven’t been able to find an answer here or on the Fly.io forums:

lib/my_app_web/plugs/robots.ex

defmodule MyAppWeb.Plugs.Robots do
  @behaviour Plug

  import Plug.Conn

  @impl true
  def init(opts), do: opts

  def call(%Plug.Conn{request_path: "/robots.txt"} = conn, _opts) do
    deploy_env = Application.get_env(:unill, :deploy_env)
    file = Application.app_dir(:unill, "priv/static/robots-#{deploy_env}.txt")


    content = File.read!(file)

    conn
    |> send_resp(200, content)
    |> halt()
  end

  @impl true
  def call(conn, _opts), do: conn
end

lib/my_app_web/endpoint.ex

defmodule MyAppWeb.Endpoint do
  use Phoenix.Endpoint, otp_app: :my_app

  plug MyAppWeb.Plugs.Robots

config/runtime.exs

deploy_env =
  case System.get_env("PHX_HOST") do
    "myapp.com" -> :production
    "staging.myapp.com" -> :staging
    _ -> :dev
  end

config :my_app, :deploy_env, deploy_env

Then, create the relevant robots-staging.txt and robots-production.txt files in priv/static/.

Remember to remove the robots.txt from the static_paths in lib/my_app_web.ex, although this plug should load and return a response before it if you’ve added it before the Plug.Static configuration.

Another option could be to have your configuration management tool write your robots.txt file. That’s what I do in Ansible. Then I just write a different robots.txt depending on which server / environment I’m deploying to and the code for it is alongside my other environment specific settings.