Just for the record, I came up with this elixir script:

[package_name, version_requirement] = System.argv()
{:ok, requirement} = Version.parse_requirement(version_requirement)

defmodule Version.Comparison do
  def unquote(:>=)(a, b) do
    Version.compare(a, b) in [:gt, :eq]
  end
end

case Hex.API.Package.get(:hexpm, package_name) do
  {:ok, {200, response, _headers}} ->
    response
    |> Map.get("releases")
    |> Enum.map(& &1["version"])
    |> Enum.filter(&Version.match?(&1, requirement))
    |> Enum.sort(&Version.Comparison.>=/2)
    |> IO.inspect(label: "Version(s) found")

  {:ok, {404, _, _}} ->
    IO.puts("Package not found.")

  {:error, reason} ->
    IO.inspect(reason, label: "Something went wrong")
end

$ mix run hex_versions.exs gen_stage "~> 1.1"
Version(s) found: ["1.1.2", "1.1.1", "1.1.0"]

Check out mix hex.outdated – I think it’s what you’re looking for.

That’s a private module, which may change at any point in time. You can probably use :hex_core’s :hex_api_package.get instead.