> It seems Ractor is still work in progress

The Ractor experimental status could almost be removed. They no longer have known bugs, and only one noticeable performance issue left (missing Ractor local GC).

But the API was just recently changed, so I think it's better to wait another years.

> I vaguely remember reading Shopify is using Fiber / Rack / Async in their codebase.

Barely. There is indeed this management obsession for fibers even when it doesn't make sense, so there is some token usage there and there, but that's it.

There is one application that was converted from Unicorn to Falcon, but falcon isn't even configured to accept concurrent requests, the gain is basically 0.

As for Rails, there isn't much use cases for fibers there, except perhaps Active Record async queries, but since most users use Postgres and PG connections are extremely costly, few people are using AR async queries with enough concurrency for fibers to make a very noticeable difference.