> When I did my 20% on Go at Google, about 10 years ago, we already had a semi-formal rule that channels must not appear in exported function signatures.

That sounds reasonable. From what little Erlang/Elixir code I’ve seen, the sending and receiving of messages is also hidden as an implementation detail in modules. The public interface did not expose concurrency or synchronization to callers. You might use them under the hood to implement your functionality, but it’s of no concern to callers, and you’re free to change the implementation without impacting callers.