There is plenty of GHC Haskell in the codebase. It's very common for banks to have proprietary in-house scripting languages. Mu, is at least based on a language standard and is not some ad-hoc design.
The codebases of Facebook and Google would be no less "ancient legacy enterprise monolith".
> It's very common for banks to have proprietary in-house scripting languages
Yeah I don't really understand this. From what I've heard, Goldman Sachs for example, have systems written in a language that no one outside of GS has used, was kinda clever in the early 90s, but hasn't really evolved since and therefore is difficult to get expertise in, and that developers working with it struggle with moving to other companies as their skills are somewhat less transferable.
Edit: at least Facebook and Google open source their internal languages/tools (Go, React, etc) so that those skills become somewhat more transferable.
