Not sure if it is still the case, but Circle used Clojure/Om [1]... when I heard them say they did that so many years ago, I knew right away it would end up being something nobody wanted to work on.

Sure enough, just checked, Om was abandoned [2] and the person blogging about how great everything was is long gone [3]. This is the absolute definition of tech debt.

[1] https://circleci.com/blog/how-circleci-processes-4-5-million...

[2] https://github.com/omcljs/om

[3] https://circleci.com/blog/why-we-use-om-and-why-were-excited...