For what it’s worth, the language itself is extremely simple and composable, which is what makes it possible for people to use it as a kind of metalanguage.
That was really popular in Rails gems around the time that Rails itself peaked in popularity, but has since died off considerably. It was never especially common outside of Rails gems. There aren’t that many things that call for DSLs.
DSLs remain very common in Ruby, but Rails taught us to avoid littering. DSLs now tend to be intentionally very tightly scoped.
E.g. Rouge, the Ruby equivalent to the Pygments syntax highlighter, defines DSLs to define lexers and themes. The do so in terms of methods on a meta-class so that the DSLs are confined to the definition of a lexer or theme class inheriting from the right classes, so there's a single place to look at the definitions, and their use don't leak all over the place.
That was really popular in Rails gems around the time that Rails itself peaked in popularity, but has since died off considerably. It was never especially common outside of Rails gems. There aren’t that many things that call for DSLs.