Then is there something within the mix tools that lets me publish multiple copies or something?

Nope.

I’m guessing I’d have to do some branching of my code to have a 1.10.x version and a 1.11.x version if I ever wanted to add additional features in newer releases?

Yup!

The most common option is to check if a given function (that is only available in the most recent release) is defined. Let’s say you want to use Enum.product/1 that will be released in Elixir v1.12.

I’d do something like this: (along with a TODO as a reminder)

def f(x) do
  enum_product(x) / length(x)
end

# TODO: remove when we depend on Elixir v1.12
if Code.ensure_loaded?(Enum) and function_exported?(Enum, :product, 1) do
  defp enum_product(x), do: Enum.product(x)
else
  defp enum_product(x), do: # custom implementation
end

Stuff like this adds code complexity and maintenance burden so it’s usually a good idea to have pretty aggressive minimum Elixir version requirement. I’d consult Compatibility and deprecations — Elixir v1.20.2 and see which version still receives bug fixes and target around that. After all, your users should be on recent Elixir versions to receive those bug fixes.