Discord uses Elixir heavily. But if you are scaling to 10s of millions of concurrent users, you're going to need those types of skills no matter the language.

I'm not using it in huge scale. But I've worked with companies that run it in production, yes.

There are a lot of Elixir products and systems out there and before those there were quite a few in Erlang. I don't have a sufficient sample size from my own experience but it is hardly old and disused or too new to be sure.

Changelog podcast runs on it. Dockyard build with it. It seems the new default for many previously Rails shops.