For what it’s worth, there’s an Elixir “backend” in the openapi-generator, introduced back in 2017, and still alive – there’s a recent PR open from an early July 2022 by @halostatue)
I tried generating Elixir app from a OpenAPI spec provided in openapi-generator’s samples. Generation works. The generated code doesn’t look like Phoenix API though. I didn’t test it further, e.g. running an actual API server to see if it behaves as expected.
It’s not exactly an answer to generating a “Phoenix API”-compatible Elixir code, but better than nothing (I guess).
On a second view, scratch what I said. It’s a client code generation 
