Depends on how you define rare. Exceedingly uncommon at the very least. stares at a pile of 150 resumes Ruby is a language languishing from being embedded in a few niches, having poor performance, and otherwise being unremarkable, imo. Ruby was made with the idea that picking it up would be easy (which it is), leading to less incentive to learn it.

Putting Ruby in the job description gives you applicants with Ruby experience, which is why the blog post was (and is) impractical. Signaling works.

After working in a dev agency that took on outside Rails projects several times while I was there, my conclusion is that Rails (not the only way to write even Web-focused Ruby, and not the only one I've used, but the only one that'll score you any points for hiring) is a pretty bad framework for any project that will have multiple teams on it over its lifetime, absent heroic technical direction & testing efforts that I've never actually seen in the wild—probably because young or outsourced teams picking Rails are doing it to move fast, over all other concerns.

You can take on or resurrect an average outside or old Rails projects. It's just slow and expensive.

Too much magic, too much room for doing things some way that the next person will never have seen before, too little grep-ability, too hard for your tools to help you.